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Vaccine Science Simplified

Twenty common questions about vaccines answered by a scientist. While this post was originally written for parents about childhood vaccinations, the information is relevant to anyone seeking to understand vaccines. The post has also been updated with some COVID-vaccine-specific information.

June, 2021


Hi there! Scientist here. I decided to write a long post about vaccines to help shed some light on how vaccines work and, hopefully, bring some clarity to topics of debate. The amount of misinformation about vaccines is pretty remarkable. And no matter what you think about vaccines, it’s tough to wade through the information about them without a background in science. FYI, my science background: BS in biochemistry and cell biology from UCSD, PhD in developmental biology from Stanford University, postdoctoral research at UCSF on the molecular underpinnings of brain development with an emphasis on a group of genes implicated in autism and other neuropsychiatric disorders, and I am now faculty at CSUS where I teach courses in molecular cell biology and developmental biology. I also have a research program focused on neurodevelopment and developmental neurotoxicology. In other words, I am 100% science nerd....and I spend much of my work life communicating topics of science to students.

I know that the vaccine debate can get heated quickly. But I think people on all sides of the argument are usually coming from a place of protection—we want to make the most protective decision for our health, our family's health, and our community's health. Decisions about vaccines are typically centered on our own personal risk assessment—our perceived risk of suffering from or spreading disease versus our perceived risk of potential side effects from vaccines. It's hard to make an appropriate risk assessment without understanding the science behind vaccines. As a mama of two, I understand the fear surrounding every medical decision we make for our kiddos. My hope—as a scientist and educator—is that this post will help equip you with information, so that you will feel empowered to make an evidence-based decision for you and your family...whatever decision that may be.

Ok, onto the fun stuff.


Q&A Contents | Complete list of questions covered below…in case you’d like to skip around instead of reading top to bottom

Q1: What are vaccines?

Q2: What are “mRNA vaccines?”

Q3: Does the COVID-19 vaccine affect fertility?

Q4: What about the other scary sounding stuff in some vaccines?

Q5-6: Why should you trust big pharma, given that they profit from vaccines?

Q7. What about adverse reactions to vaccines?

Q8: If not vaccines, what causes autism?

Q9: What is thought to cause the allergic reaction to the COVID-19 vaccine?

Q10: What is herd immunity?

Q11: Isn’t natural immunity better than vaccine-induced immunity?

Q12: Why do some vaccines not give lasting immunity?

Q13: How long will COVID-19 and/or COVID-19 vaccine-induced immunity last?

Q14: What is the difference between the FDA “Emergency Use Authorization” and FDA “full approval?"

Q15: Why do people often get fevers after being vaccinated?

Q16: What’s up with vaccine shedding?

Q17: Should adults get vaccinated for things like measles, too?

Q18: Could a vaccine or booster given to a breastfeeding mama give the baby passive immunity through antibodies present in the breast milk?

Q19: If newborns get passive immunity from mama during pregnancy, why are they susceptible to illness?

Q20: What about the alternative vaccine schedule versus the CDC recommended vaccine schedule?


Before we dive in, some important vocabulary:

  • Pathogen = Disease-causing bacteria or virus

  • SARS-CoV-2 = Virus in the coronavirus family that causes COVID-19 (stands for Severe Acute Respiratory Syndrome COronaVirus 2)

  • COVID-19 = Disease caused by SARS-CoV-2 (stands for COronaVIrus Disease of 2019, after the virus family and the year the disease first emerged)


Q1: OK, so what are vaccines?

A: ­­­Usually, vaccines are viruses or bacteria that have been modified so they cannot hurt you, but still look like pathogens to your immune system. That part is key. When a weakened pathogen (or “acelluar” pieces of a pathogen) enters your body, your immune system responds by making antibodies that will bind specifically to that pathogen and target it for destruction.

Here’s the really cool part—our immune system makes cells called memory T cells and memory B cells that will stay in our body for a really long time (depending on how strong the vaccine is). Memory T cells function to help trigger a number of immune responses, including activating antibody-producing B cells. Memory B cells are primed to make antibodies specific to each pathogen. This is important because our immune response can take a long time—long enough for pathogens to have debilitating and sometimes lethal consequences. But if you have these memory B cells ready to go—that have already been trained to make antibodies specific for the pathogen in question—the cells will kick into action right away and make antibodies that will get rid of the pathogen before it hurts you.

Q2: What about “mRNA vaccines?”

A: The BioNTech/Pfizer and Moderna COVID-19 vaccines are both mRNA vaccines. In this case, a specific quantity of mRNA (messenger RNA) is introduced to your cells. To be clear, this mRNA cannot integrate into your DNA or alter your genetic code. The mRNA in these vaccines simply provides cells with temporary instructions for making a part of SARS-CoV-2 (the virus that causes COVID-19), called the “spike protein.”

The spike protein is the part of the virus that binds to receptors on your respiratory and vascular (vessel) cells. But without a virus attached, the spike protein created in response to the vaccine should not hurt you. Think of the spike protein as the key to your house. On its own, the key doesn’t really do anything. But if you were to give that key to a group of hungry teenagers in search of food and screens…well, now you have a problem on your hands. In this ridiculous metaphor, the virus that causes COVID-19 is equivalent to those teens with the key. But the mRNA vaccine is just the key on its own. Just the key = your house is intact. As is your body with just the spike protein by itself because there is no virus for it to provide access to.

My lock/key metaphor might not apply if there was an incredibly high amount of spike protein present. The COVID-19 vaccines cause a tiny amount of spike protein to be produced, just enough for your body to detect it and mount an immune response. There have been a few studies in which scientists introduced very high levels of the spike protein into mice or rats and it caused damage to vessels. But as I discuss in Q4 below, DOSE MATTERS! These studies used levels of spike protein that were more similar to what might be observed in a COVID-19 disease state or beyond, NOT what is observed following vaccination. It can be hard to extrapolate doses in rodent models to humans, but one professor of pharmacokinetics estimated that it would require about 60,000 doses of vaccine injected simultaneously into one human to induce the levels of spike protein used in those studies. For a more comprehensive discussion on this specific topic, here is a nice discussion with references.

Another important detail is how unstable mRNA is. Once introduced to the body, the mRNA will only persist for a matter of days before being degraded (this is why those mRNA vaccines have to be stored at super cold temperatures—because mRNA is very unstable). Similarly, the spike protein will also be broken down by your body. But in the short time that it is present in your body, the immune system will recognize it as foreign and mount an immune response that teaches the body how to create specific antibodies that know how to recognize the SARS-CoV-2 spike protein. That way, if you are ever infected, you will have memory B cells trained to make those spike-protein-specific antibodies ready to go to battle against the virus right away! This is absolutely critical for preventing the respiratory and vessel damage that occurs when people are infected with SARS-CoV-2, a virus that can just wreak havoc on our bodies.

Q3: While we're on the topic of the COVID-19 vaccine, does it affect fertility?

A: First, I think it is imperative that we test for any kind of aberration caused by any medical treatment. We want to prevent or treat diseases and, to the best of our ability, not create new disorders or tissue damage with those treatments. This is the reason for extensive clinical trials and oversight by regulatory agencies. So, when I started getting questions from friends about COVID-19 vaccines and fertility, I wondered if this was an effect that was observed in the clinical trials—but it is not.

The notion that the vaccine might affect fertility arose when someone put forth the idea that the SARS-CoV-2 spike protein might have a similar structure/shape to a human protein called syncytin-1. Syncytin-1 is a protein that plays an important role in fertility. If these two proteins did share a similar structure, then antibodies produced to attack the viral spike protein might also attack cells that have syncytin-1 on their surface.

As far as I can tell, this idea was suggested because of a slight similarity between syncytin-1 and the spike protein of SARS-CoV-1 (not SARS-CoV-2). But here’s the thing— syncytin-1 and the spike protein of SARS-CoV-2 are VERY different. Here is a reference to a paper that discusses this topic and also includes a comparison of the two protein sequences [1]. Out of curiosity, I also did an alignment of these two protein sequences on my own and saw the same thing—the proteins have such different sequences that it is incredibly unlikely an antibody designed to target one would also be able to target the other.

In addition to the fertility argument falling flat based on protein sequence differences, there is also data from the Pfizer/BioNTech vaccine clinical trial indicating that fertility is not impacted. During the phase 3 clinical trial, 23 people became pregnant after receiving the vaccine. One women did suffer a miscarriage, but she was in the placebo group and, therefore, had not received the vaccine.

Q4: What about the other scary sounding stuff in some vaccines?

A: The intended purpose of the other components are to ensure the vaccine stays safe and effective. And while they sound awful, they are all actually safe in the amounts present. For example, formaldehyde sounds scary, but did you know that it is a normal metabolic byproduct that your body produces in small amounts constantly? You produce more formaldehyde over a matter of minutes than you get from a vaccine. Another fun fact: there is 4-15 times more formaldehyde in a single apple than any one vaccine. And your body simply processes it and gets rid of it (again, it knows how since you are always producing it). Aluminum? Present in things ranging from organic pears to natural breast milk.

One of the first things biochemistry students learn is that dose matters. Yes, large amounts of aluminum and formaldehyde are bad…but large amounts of water can be lethal. Oh, and mercury-containing thimerosol is no longer in early childhood vaccines because it was removed due to public outcry. However, there is still zero scientific data to suggest that the doses of thimerosol previously found in those vaccines has any detrimental effects. In fact, the type of mercury in thimerosol is ethyl mercury, which is readily flushed from the body. The bad mercury that our body has a harder time getting rid of is methyl mercury (found in tuna).

Q5: Why should you trust big pharma, given that they profit from vaccines?

A: My first answer is that you don't have to. There are a lot of scientists who have published research on the safety of vaccines that are not affiliated with big pharma and do not profit from the results of their findings. They are people like me—who became scientists because they wanted to help learn more about biology in order to diminish human suffering. We work for academic institutions, not big pharma. We ask questions without a vested interest in the answers. These are the scientists that can provide you with unbiased information.

You can search for research studies yourself on Pubmed, the largest database of scientific journals (on the first page of published studies, researchers are required to list any conflicts of interest):

As an example, you will find that when you search for studies on autism and vaccines, of the hundreds of studies conducted, there is still no scientific data to suggest a link between the two.

Q6: But, still, big pharma makes SO MUCH money...

A: Yes, they do. They make money on every drug they produce. I have opinions on big pharma’s business practices that I won’t go into now because it actually has nothing to do with the argument about vaccine effectiveness or safety. For better or for worse, our entire medical system is profit based (right along with our entire economy). The people at the forefront of the anti-vaccination movement also make a lot of money. That is not why I don’t believe them, though. I don’t believe anti-vaccination proponents because of the absence of scientific data to support their claims. As a scientist, I only believe what the scientific data supports. I read research, not opinions. (That is not meant as a slight to anyone! I am simply stating my practices. I know that reading primary research papers can be like reading a different language if you do not have a science background, so I would not really expect any non-scientist to have this practice. It’s the same reason I don’t read economics papers. No thanks.)

Q7: What about adverse reactions to vaccines?

A: Vaccines are the most effective preventative medicine in existence—they have saved millions and millions of lives (a recent study estimated 37 million lives were saved by vaccines between the years 2000 and 2019 alone). Vaccines trigger immune responses in our bodies, so that we will be protected from dangerous pathogens that cause devastating diseases, like tetanus, polio, whooping cough, and COVID-19. Immune responses are powerfully protective. But whether the response is to an actual pathogen or to a vaccine, a powerful immune response can also have negative outcomes.

First, the overall risk of adverse reaction to a vaccine is something like 0.003%. The majority of those 0.003% have been minor allergic or inflammatory reactions. Severe allergic reactions (like anaphylaxis) can occur, though these reactions are extremely rare and are typically treatable with epinephrine. There have also been cases of autoimmune disorders being triggered by vaccines. Autoimmunity is when your body produces antibodies that attack and damage your own cells instead of the pathogen. We have molecular checkpoints that normally prevent this from happening. Autoimmune disorders typically occur when these checkpoints fail. Sometimes it is not entirely clear whether vaccines are actually the trigger because autoimmune events can be provoked by an actual pathogen, too. There have also been instances of vaccine-induced fever triggering symptoms in people who have rare genetic defects in their mitochondria (a cellular organelle)—but we now understand that any fever would trigger symptoms in those individuals, whether from a vaccine or a common cold. But the truth of the matter is that even though severe adverse reactions are rare and sometimes not necessarily attributed to the vaccine, to the people afflicted it doesn't matter how rare the reactions are in the general population—because they have experienced that tragedy.

For individuals with a family history of autoimmunity or allergies, it is important to have a conversation with a physician about whether that family history increases the risk of an adverse event following vaccination. These individuals will need to weigh the risk of disease versus vaccine-associated side effects. If they opt to not get a vaccine, they will need to be more cautious in order to avoid infection. Again, because infection can also lead to an immune response that triggers autoimmune dysfunction. Another important reason for these individuals to avoid infection—from the perspective of public health and being community-minded—is to avoid serving as a disease vector by enabling the virus to proliferate in them and potentially enable the virus to go on to infect others who may suffer and/or die.

People who are immunocompromised, meaning they have a weakened immune system (chemotherapy patients, HIV patients, genetic immune deficiencies, etc.), often cannot be immunized because their immune systems are so weak that even the weakened virus might hurt them.

For people who are unable to get vaccinated for health reasons, accomplishing herd immunity within the rest of the community is critical to keep those who are more vulnerable safe. What is herd immunity, you ask? That is the topic of Q10!

Q8. If not vaccines, what causes autism?

A: I have been studying the molecular underpinnings of autism and brain development for about 10 years, so I'll give a brief summary of what we know. First, while vaccines can cause rare adverse events associated with allergic reactions and autoimmunity (as discussed above), there is no credible evidence that vaccines cause autism spectrum disorder (ASD).

ASD is a group of diverse neurodevelopmental disorders that usually begin in utero and are influenced by both genes and environmental factors. Though the changes in the brain often begin in the womb, the behavioral differences that define ASD generally do not manifest until the age of 2 or 3, which is when diagnosis usually occurs. ASD symptoms range from mild to severe. Many people with mild-moderate ASD do not need treatment and are certainly not seeking a "cure"—rather, they positively contribute to the neurodiversity of our communities. But some individuals with severe forms of ASD have great challenges communicating, exhibit what is called restrictive and repetitive behaviors, and can have extreme sensitivity to the world around them—all of which can make it hard for them to thrive or live independently.

So what do we know about the causes? Again, many genes and environmental factors associated with ASD have been identified. Of the autism risk genes that have been identified, most affect how the brain develops during gestation or how brain cells communicate. A couple of recently published papers delineated specific mutations in a large number of candidate ASD risk genes [2, 3]. For more information, here is a nice write-up on the genetics of ASD. There has also been research showing the influence of different environmental factors. One example of an environmental factor associated with autism is maternal autoantibodies—these are naturally generated by some pregnant mamas and present in the womb. Scientists at the UC Davis MIND Institute are working hard to better understand what causes the production of these autoantibodies, which impair brain development by attacking fetal brain proteins and are attributed to up to 20% of ASD cases [4]. Another contributing environmental factor is high levels of inflammation in response to infection in the pregnant mama during the first trimester (further discussed here)—a situation that can at least be partly mitigated by being vaccinated prior to pregnancy. Other environmental factors associated with autism have ranged from anti-epileptic medication (valproate) to pesticides (chlorpyrifos). But despite of an overwhelming amount of time, money, and energy devoted to investigating a potential link between autism and vaccines—certainly more than has been invested into any other environmental factor—every study has come back with the same results: there is no credible or definitive data to suggest a link between autism and vaccines. Just one example: every epidemiological study conducted on populations of children living in the same community has shown autism occurs at the same rate in vaccinated and unvaccinated children, even in higher risk populations.

As a person who studies developmental neurotoxicology (i.e, the study of chemicals that impair brain development), who is continually shocked by the thousands of chemicals in our environment that have undergone little to no toxicological testing—and who has pored over the vaccine-autism studies—it is actually a source of great confusion and frustration that there is not more alarm given to other prevalent environmental chemicals, like plastics, flame retardants, and organophosphate pesticides. These are classes of molecules that have been shown time and again to influence prenatal brain development and have been linked to neurodevelopmental disorders, like autism. To me, it feels like people still focused on the idea that vaccines cause autism are just paying attention to the wrong thing.

Q9: What is thought to cause the allergic reaction to the COVID-19 vaccine?

A: mRNA vaccines are delivered in a little lipid droplet (a “lipid nanoparticle”) that is similar in composition to our own cell membranes. This property allows it to fuse with our cell membranes to deliver the mRNA. The allergic reactions to COVID-19 mRNA vaccines are thought to be caused by something called polyethylene glycol (PEG) [5], which helps stabilize the lipid nanoparticle. Allergic reaction to PEG is rare—these compounds are largely regarded as non-toxic and are also found in toothpaste, mouthwash, ointments, skincare products, etc. The small chance of allergic reactions is the reason you are asked to wait for 15-30 minutes before leaving your medical provider after receiving a COVID-19 vaccination. If an allergic reaction were to occur, it would likely happen within this time frame. Medical personnel would then administer epinephrine to mitigate the reaction.

Q10: What is herd immunity?

A: It’s kind of basic math. Viruses cannot replicate on their own. They need to infect a host cell in order to replicate. If they don’t make it into a host cell, they will eventually die. Here's an easy example: a person infected with a virus walks into a room where there are 20 vaccinated people separating him from a single unvaccinated person. That virus cannot move from the infected person and replicate in any of the vaccinated people because once it gets into their bodies, those memory B cells start pumping out antibodies that kill it before it can replicate and spread. Therefore, those 20 vaccinated people make it harder for the virus to make it to the single unvaccinated person. If half of the people were unvaccinated, that virus would get to have a replication party in all of their cells and would have a much easier time surviving, multiplying, and spreading. Herd immunity is just a basic principle about how infectious pathogens spread. If someone tells you it doesn’t exist, you should be wary of any other scientific information they give you because it means that they have never taken or studied immunology or microbiology and are not qualified to have an educated discussion about those topics.

The tricky thing about vaccines and herd immunity is that herd immunity really only works when a high percentage of the population is vaccinated. Otherwise, viruses have an easier time spreading around our communities, putting our neighbors at risk who cannot be vaccinated (newborns, people battling cancer, etc.), and who are also much more likely to die as a result of infection. That is why the scientific community is so scared. We feel that even a single death from a vaccine-preventable disease is a tragedy.

Q11: Isn’t natural immunity better than vaccine-induced immunity?

A: Well, sometimes the immune response is stronger because the pathogens are not weakened. So, if you make it through the illness, you should have a great supply of those memory B and T cells. The problem is that a lot of these vaccine-preventable pathogens can cause blindness, deafness, brain damage, paralysis, or death. I know of a mama who has a sister who contracted rubella while she was pregnant. Her baby was born blind and deaf because of the infection. So, yes, she now has great immunity to rubella. But she would give anything to have had vaccine-induced immunity prior to her pregnancy.

There are also instances of mild infections by pathogens that do not necessarily cause a robust immune response. In that case, the vaccine may actually trigger more effective and sustained memory B and T cells.

Q12: Why do some vaccines not give lasting immunity?

A: Each vaccine has a varying degree of effectiveness. By effectiveness I specifically mean the quantity and quality of memory cells that will stick around in the immune system post-vaccine. For example, the smallpox vaccine gave immunity for 65 years whereas the pertussis vaccine only lasts for about 10 years. This is the purpose of boosters. Boosters essentially tell your immune system that it is still important to mount a defense against the pathogen, and replenishes your stock of memory cells.

Q13: How long will COVID-19 and/or COVID-19 vaccine-induced immunity last?

A: Because the SARS-CoV-2 virus itself and COVID-19 vaccines are so new, we have no way of knowing the definitive answer yet. The only way to know this is to monitor immunity in previously infected and vaccinated individuals. The good news is that a recent study indicated SARS-CoV-2-specific memory B cells can be detected for at least a year after infection [6]. In this study, the most robust long-term immunity was observed in individuals who had COVID-19 and were also subsequently vaccinated. Individuals who had been vaccinated but not had COVID-19 also showed robust long-term immunity. Again, in this case “long-term” is only a year, but scientists are hopeful that the immunity will persist for multiple years. (Immunity to SARS-CoV-1—the virus that causes SARS and closest virus relative to SARS-CoV-2—can last up to 17 years!) But we may very well end up needing booster shots to remind our immune systems to keep those memory cells primed and ready to fight SARS-CoV-2!

Q14: I heard that the COVID-19 vaccines have “Emergency Use Authorization” from the FDA, but not “full approval.” Why? And what is the difference?

A: The difference between these authorization types is also the “why.” The FDA grants “Emergency Use Authorizations” (EUAs) during emergencies in which they need to expedite their normal, relatively slow approval process in the interest of public health. Two important things to know: (1) even with EUAs, the FDA still requires significant large-scale clinical trial data showing convincing efficacy and safety profiles, and (2) the FDA still requires longer-term data in order to grant full approval.

For the COVID-19 vaccines, EUAs were granted to the vaccines currently in use after three months of phase 3 trial data, which included anywhere from 20,000-40,000 trial participants. The FDA examined efficacy, safety, as well as the ability for safe and consistent vaccine manufacturing and distribution. Their EUA assessment was very rigorous, but participants were only followed for two months post-vaccination when the EUAs were applied for. For full approval, the FDA needs at least six solid months of data…and then it will take at least another six months for the FDA to review all of that data—for a grand total of at least 12 months to achieve a full approval designation. So, the big difference is that volunteers are observed for a longer period of time in order to measure (1) the duration of protection, and (2) if there might be rare adverse events that occurred down the road. That being said, when adverse reactions to vaccines occur, they typically manifest within the first few weeks since these reactions are usually the result of an autoimmune dysfunction (would occur within days/weeks) or an allergic reaction (would occur pretty immediately—likely within fifteen to thirty minutes, but up to a few hours). This is why I am personally very comfortable with a vaccine that has been granted an EUA. But our regulatory agencies must be as cautious as possible when considering the risk-benefit metric for the entire population, so I am also a big fan of the requirements for full approval.

Ok, to summarize—despite being an expedited form of authorization, EUAs still require large-scale phase 3 clinical trial data that demonstrate convincing safety and efficacy profiles, as well as reliable manufacturing and distribution plans. The EUAs were granted after following trial participants for two months after their full vaccination regime, whereas full approval requires a six-month follow-up.

Q15: Why do people often get fevers after being vaccinated?

A: Part of the natural immune response is the release of molecules called chemokines, which cause fever. As a mama, I know how scary it can be when your little one has a fever, but a post-vaccine fever is indicative of a strong immune response and means they are making great memory B cells. That does not mean you shouldn’t treat your baby’s fever! (Please consult your pediatrician on when you should treat your baby’s fever.)

One of the physiological reasons for fever is that neutrophils (white blood cells involved with our immune response) actually work optimally at higher temperatures. I don’t view fevers or feeling tired as vaccine “side-effects”—I view them as “the main effects.” A post-vaccine fever and lethargy are signs that your immune system is working hard for you!

Q16: What’s up with vaccine shedding?

A: Vaccine shedding is something only possible with a live attenuated virus. This is different from the pertussis vaccine, for example, which is an acellular vaccine, meaning it contains various pieces of the pertussis bacterial molecules and is not infectious at all, cannot cause illness ever, and cannot shed. Again, a live attenuated virus is a weakened virus that reproduces so slowly that a normal immune system will take care of it before it causes any harm. If a person is immunocompromised, live attenuated vaccines cannot be used because their immune system might not be able to handle even a weakened virus. The nasal spray flu vaccine does have a risk of vaccine shedding because the vaccine is administered directly to the mucus membranes of the nose. Therefore, if that recently immunized person were to sneeze onto an immunocompromised person, there is a theoretical possibility that the attenuated virus could give that immunocompromised individual the flu. This is why it is recommended to stay away from immunocompromised individuals for a week after getting the nasal spray flu vaccine. Other live attenuated viruses are injected into muscle. Some of the weakened virus will get into the lymphatic system, which is where all that good immunity will happen (production of specific antibodies, effector cells, and memory cells that will stay around for a long time). From there, some of the vaccine can enter saliva and mucus, although it is going to be a much lower amount. I think this is why the CDC only has the recommendation to steer clear of immunocompromised individuals in the case of the nasal spray flu vaccine. BUT, and this is critical, the virus that would potentially be shed post-vaccine is the attenuated (weakened) virus that does not cause illness in a person with a normal immune system. This is why vaccine shedding does not cause disease EVER in a person with a normal immune system. It would essentially be like getting an ultra-tiny dose of a vaccine (not enough to even cause an appreciable immune response that would lead to acquired immunity). This is anecdotal, but when my daughter was newborn, my husband did not realize this about the nasal spray flu vaccine when he took our 2-year-old to the doctor…and he got him the nasal spray form of the flu vaccine. I’m happy to report that my newborn daughter did not get the flu. I actually wasn’t really worried; it’s a very minimal risk….but when a person is severely immunocompromised it is important to worry about any potential risk.

Q17: I heard a lot of adults are to blame for the last measles outbreak. Should adults get vaccinated for things like measles, too?

A. Absolutely! If you are unsure of your immunity, you can talk to your medical provider about checking your titer (a measure of your immunity), or you could just get a booster.

Q18: If I have a baby that is too young for a vaccine, could a vaccine or booster given to a breastfeeding mama give the baby passive immunity through antibodies present in the breast milk?

A: Passive immunity is the transfer of active antibodies from one person to another. This happens during pregnancy when antibodies present in mama cross the placenta to the developing fetus. When my daughter was a baby, I spoke to an immunologist friend about passive immunity through breast milk because I was considering getting the MMR booster to potentially help my baby girl during the measles outbreak. But he said (sadly) it probably would not boost her passive immunity an appreciable amount (for a virus as strong as measles, anyway). There are five classes of antibodies (IgA, IgG, IgD, IgE, and IgM). The type that is most effective in preventing infection from something like the measles is IgG. These antibodies cross the placenta during pregnancy and give passive immunity to the baby when it is newborn. The primary type of antibody that gets into breast milk is IgA. It provides some protection, but it’s just not as great as IgG.

Q19: If newborns get passive immunity from mama during pregnancy, why are they susceptible to illness?

A: Passive immunity only lasts for a short time. That’s because antibodies tend to not survive very long (a few weeks to a few months, on average). Unfortunately, the effector cells and memory cells that are responsible for making the antibodies in mama do not cross the placenta. The memory cells are the cell types that stick around for years to provide lasting immunity. I read a study that indicated 88 percent of babies of vaccinated mothers have passive immunity to measles at 4 months, and that number dropped to 15 percent by 8 months of age [7]. Although, and this is important, the amount of antibodies acquired through passive immunity may not be sufficient to protect the baby from a strong pathogen.

Q20: What about the alternative vaccine schedule versus the CDC recommended vaccine schedule?

A: I've never found evidence to support the alternative vaccine schedule. It is my understanding that it is something to make parents feel more comfortable. There are a lot of factors taken into account for the CDC schedule, which have to do with considerations like when the acquired immunity will be best. For example, MMR is not given until 12 months because they want to make sure that all passive immunity acquired from mama during pregnancy is gone by the time the vaccine is administered because those circulating antibodies would decrease the immune response to the vaccine. So, MMR can be given at 6 months, but is better at 12 months...and I recently read a study indicating even a little tiny bit better at 15 months [8]; but, you could possibly do the initial shot earlier than 12 months and then get the booster early if you are concerned about measles in your community (of course, talk to your doc about these decisions).


In Closing

I hope this is helpful! Again, I have no financial interest in this debate. As the mama of two precious kiddos, and as the stepdaughter to a wonderful man who spent his final 9 months severely immunocompromised due to chemotherapy, I am certainly emotionally invested. But as a scientist and educator, my primary motivation is to help spread knowledge and quell fear in order to help empower people to make informed decisions.


For links to more information about vaccines please check out this post:



1 Kloc M, Uosef A, Kubiak JZ, Ghobrial RM. Exaptation of Retroviral Syncytin for Development of Syncytialized Placenta, Its Limited Homology to the SARS-CoV-2 Spike Protein and Arguments against Disturbing Narrative in the Context of COVID-19 Vaccination. Biology (Basel). 2021 Mar 19;10(3):238. doi: 10.3390/biology10030238. PMID: 33808658; PMCID: PMC8003504.

2 Iossifov I, et al., The contributions of de novo coding mutations to autism spectrum disorder. Nature. (2014) 515(7526)

3 De Rubeis S, et al., Synaptic, transcriptional and chromatin genes in autism. Nature. (2014) 515(7526)

4 Bauman MD, et al., Maternal antibodies from mothers of children with autism alter brain growth and social behavior development in the rhesus monkey. Transl Psychiatry. (2013) 9;3

5 Sellaturay, P., Nasser, S., Islam, S., Gurugama, P. and Ewan, P.W. (2021), Polyethylene glycol (PEG) is a cause of anaphylaxis to the Pfizer/BioNTech mRNA COVID-19 vaccine. Clin Exp Allergy, 51: 861-863.

6 Turner, J.S., Kim, W., Kalaidina, E. et al. SARS-CoV-2 infection induces long-lived bone marrow plasma cells in humans. Nature (2021).

7 Hinman A., et al., Comparison of Vaccination with Measles-Mumps-Rubella Vaccine at 9, 12, and 15 Months of Age. J Infect Dis. (2004) 189

8 De Serres, et al., Passive immunity against measles during the first 8 months of life of infants born to vaccinated mother or to mothers who sustained measles. Vaccine. (1997) 15(6-7):620-3.


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