COVID-19 Vaccine Boosters: Safety, Efficacy, and Improving Uptake

ContagionContagion, September 2022 (Vol. 07, No. 4)

“Vaccines are the most highly scrutinized public health interventions we know,” said Donald Alcendor, PhD.

Watch the Peer Exchange series, "COVID-19 Expectations on the Roll-out of Booster Shots," here.

COVID-19 vaccine boosters appear to have a similar safety profile as the initial series and appear to reduce the risk for severe disease, hospitalization, and mortality related to COVID-19 infection. However, the rare adverse reactions tend to be overrepresented in the media.

In a recent Contagion® Peer Exchange, moderated by Peter L. Salgo, MD, infectious disease experts discussed the safety and efficacy of COVID-19 boosters and strategies to improve uptake among individuals who are vaccine hesitant.


Adverse effects from the booster shots appear to be similar to those with the initial doses or series, although the reactions vary among individuals, and a serious adverse event could occur with any of the vaccines, Donald Alcendor, PhD, said.

However, Salgo noted that unlike past rollouts of vaccines, a single adverse reaction can become front-page news without the context of the actual incidence of severe reactions. “Whether it’s clots, neurologic complications, [or] myocarditis, [the general public doesn’t] necessarily have the statistical background to evaluate this is 1 in 1,000,000, 1 in 10,000,000, 1 in 100,000,” Salgo said.

Alcendor said that explaining to potential vaccine recipients about the safety of other vaccines and the rigorous process they go through to obtain approval is important to provide context for the overall safety of the COVID-19 vaccine. “I explain to parents their experience with vaccines, and they have quite a bit. I ask them, ‘Do you have a child that goes to public school?’ They tell me yes, [so] I say, ‘To go to public school, there [are] state-mandated vaccinations they have to have, and they [must] be updated every year.’ I [also] ask them, ‘How many of your children come back after receiving a vaccine and complain of an adverse event?’ I don’t hear anything. The reason being is that vaccines have been safe and effective for decades,” he said.

Alcendor added that vaccines also go through an extensive process to obtain approval. In the United States, this includes an investigational new drug application, 3 phases of clinical trials, a biologics license application, inspection of the manufacturing facility, presentation of the results to the FDA Vaccines and Related Biological Products Advisory Committee, usability testing of product labeling, and pharmacovigilance trials after approval to ensure safety.1 “Vaccines are the most highly scrutinized public health interventions we know,” Alcendor said.

Jeff Goad, PharmD, MPH, added that information about vaccine adverse effects is often obtained from anecdotal reports on social media, where adverse reactions tend to be overrepresented. “One of the things we must do is [encourage] patients who took their 5 [or] 6 year old [and] didn’t say anything— their arm didn’t even hurt—to [post that experience on] social media. Because guess what’s showing up there? ‘My arm hurt. I got some weird reaction that people associate with vaccine.’ That’s all [people are] seeing. Therefore, we must encourage our patients [to] post this stuff on social media: ‘My kid got vaccinated, it was fine.’”

Salgo and Angela Rasmussen, PhD, also added that individuals were more likely to accept a higher level of risk and adverse events from polio and smallpox vaccines several decades ago because they had observed the harmful effects of the diseases caused by the viruses. “When you don’t grow up seeing your friends paralyzed, seeing your friends in an iron lung, [or] seeing children all over the place getting sick and sometimes dying from a preventable illness, you start to get a little complacent,” Rasmussen said.

The novelty of the SARS-CoV-2 virus, which was initially transmitted zoonotically to the human immune system, has resulted in a high level of mortality from COVID-19, Alcendor explained. “We look at the Spanish flu, where young people had not seen that virus, and [it] had a tremendous amount of mortality in younger folks [but] not in younger folks [who] had seen that virus prior. It has to do with your immune recognition of something. If your immune system has something to go on, then it’s going to be better equipped to fight it. We had nothing to go on with [COVID-19],” he said.


The efficacy and safety are essentially similar across the 3 manufacturers of COVID-19 booster shots (Pfizer-BioNTech, Moderna, and Johnson & Johnson), according to Alcendor. “[When] we think [about] what a vaccine should do— prevent severe disease, hospitalizations, and death in [individuals who] receive it—there’s no difference between those 3 vaccines,” he said. Goad said the adverse event profile for Pfizer’s booster appears similar to that of the second dose of the initial series, which had more reported adverse events than the first dose, and he expects these findings to be similar with the other vaccines based on anecdotal reports.

Individuals who are pregnant are a highly undervaccinated population and have a high risk for complications related to COVID-19, so ensuring this population is up-to-date on COVID-19 vaccinations is important, Jason Gallagher, PharmD, FCCP, FIDP, FIDSA, BCPS explained. “Some of the worst outcomes I’ve seen personally have been in pregnant women, and they’re heartbreaking,” he said. Alcendor added that data from the Moderna trial (NCT04958304) of women who became pregnant after receiving the COVID-19 vaccine showed no adverse effects, and Moderna studies of a rat animal model found no changes in fertility, number of pups birthed, or changes in pregnancy, further supporting that the vaccine was safe in this population.2

Rasmussen added that no increase in miscarriage or stillbirth was found among those who were pregnant and vaccinated compared with unvaccinated counterparts.2 Alcendor also added that the American College of Obstetricians and Gynecologists and all other governmental bodies for health and wellness of women have approved vaccination in those who are pregnant and breastfeeding.


Goad noted that the CDC is tracking the number of individuals who have received at least 1 dose, are fully vaccinated (defined as completion of the primary series), and have received a booster dose.3 Gallagher added that reducing the proportion of individuals who have not received any vaccines is considerably more important than widespread uptake of booster shots among those who received an initial vaccine series.

Goad said that a key shortcoming of the current surveillance system is that vaccination status, hospitalization, and severe disease are not evaluated prospectively and tend to exist in separate silos. However, data are promising regarding the ability of vaccine boosters to reduce severe disease in certain populations and possibly infection rate, according to Gallagher.

Early data from Israel in 2021 found that compared with those who received only the initial vaccine series, patients 16 years or older who received a booster dose of the BNT162b2 (Pfizer-BioNTech) messenger RNA vaccine at least 5 months after their initial series had a lower rate of confirmed infection by a factor of approximately 10 across 5 age groups (range among age groups, 9.0-17.2). Boosting also lowered the rate of severe illness by a factor of 17.9 (95% CI, 15.1-21.2) in the age group of 60 years and older and 21.7 (95% CI, 10.6-44.2) in the 40- to 59-year age group.4 Gallagher noted that severe disease is the “true end point of concern” and should be studied further, with respect to booster efficacy.


1. Centers for Disease Control and Prevention. Vaccine Testing and the Approval Process. Updated May 1, 2014. Accessed July 20, 2022.

2. Food and Drug Administration. Vaccines and Related Biological Products Advisory Committee Meeting, FDA Briefing Document, Moderna COVID-19 Vaccine. Revised December 17, 2020. Accessed July 20, 2022.

3. Centers for Disease Control and Prevention. COVID Data Tracker. Updated July 21, 2022. Accessed July 23, 2022.

4. Bar-On YM, Goldberg Y, Mandel M, et al. Protection against Covid-19 by BNT162b2 Booster across Age Groups. N Engl J Med. 2021;385(26):2421-2430. doi:10.1056/NEJMoa2115926

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