Few measures in the history of public health intervention compare to the impact vaccines have had on preventing disease and saving human lives. In the United States, vaccination against nine common vaccine-preventable diseases has reduced them by more than 90%, with some diseases now eliminated. In a cohort of vaccinated children, nearly 20 million cases of diseases were prevented, including preventing over 40,000 deaths. Yet for many, whether to seek an approved COVID-19 vaccine is still a question.

To that end, I want to address six common questions regarding COVID-19 vaccines:

1. Can I get COVID-19 from the vaccine?

The vaccines currently available in the U.S. (Pfizer/BioNTech and Moderna) work by employing messenger RNA (mRNA). In these vaccines, the delivered mRNA instructs cells to produce only virus “spike” proteins, which are normally found on the viral surface. Immune cells process and present the “spike” protein to additional immune system cells, which induce an immune response, including the production of antibodies. The vaccines mimic a COVID-19 infection, but since no live virus is used, there is no way of becoming infected with COVID-19. Furthermore, these vaccines cannot alter one’s DNA, as the viral mRNA is ultimately eliminated and our cells do not convert mRNA to DNA.

2. What do we know about the vaccines’ efficacy?

Both vaccines are extraordinarily effective. In large, clinical randomized placebo-controlled trials, the vaccines lowered the probability of developing COVID-19 by approximately 95% versus placebos. These results are very impressive since many vaccinations are typically 50% to 70% effective. Furthermore, in those individuals who were vaccinated, but still contracted COVID-19, disease severity was greatly reduced. These studies enrolled participants from diverse populations, representing various ages, sexes and ethnicities. Finally, though both vaccines require two doses, substantial protection was conferred after around two weeks of initial doses. To achieve 95% efficacy, a second dose, which boosts the immune response, is required.

3. How long will the vaccines work?

Because COVID-19 vaccination data has been available only since summer 2020, the answer to this remains unclear. However, data from phase 1 trials of the Moderna vaccine indicated that neutralizing antibodies persisted in the body after four months, with antibody concentrations only declining slightly from peak concentrations.

4. Do the vaccines prevent transmission of the virus to others?

This is a difficult question to answer with precision. Certainly, this is a valid concern, as an estimated 40% of those infected with COVID-19 do not display obvious symptoms, but may still transmit the virus. It is not uncommon for some vaccines to continue permitting asymptomatic virus transmission. This may also be true for COVID-19 vaccines. Moreover, since it can take up to two weeks to build sufficient immunity against the virus, it is recommended to continue to practice social distancing, wearing masks, hand washing and avoiding crowded indoor settings after receiving the initial vaccine. Until some level of widespread population protection is achieved, continuing these hygienic practices may be appropriate.

5. What is herd protection and how is it achieved?

Herd protection or immunity occurs when a pathogen encounters a large number of hosts that are immune to infection and as a result, cannot spread widely among the population. Once an adequate quantity of the population is no longer susceptible to infection, the probability of the pathogen encountering a susceptible host becomes sufficiently small. When the pathogen is unable to propagate in a susceptible host population, the pathogen eventually fades to baseline levels. Herd protection is the desirable outcome of large-scale vaccination campaigns, where high levels of vaccination-induced immunity occur. In this manner, the immune population “protects” those who may remain susceptible — such as people with compromised immune systems — to infection by the pathogen. Vaccine protection is never 100%, but the prospect of reducing transmission of COVID-19 through vaccination is excellent.

6. What about vaccine safety?

Both vaccines appear safe, but no vaccine or medical procedure is 100% safe. The most common side effects are injection site pain, fatigue and headache, with high fevers less commonly reported. These side effects typically resolve within 48 hours. Reports suggest that side effects are more common in younger recipients, with second shot administration also inducing side effects in some recipients. Certainly, any harmful effects are much less of a risk than becoming sick with COVID-19.

As this article went to print the U.S. Food and Drug Administration approved a third vaccine, manufactured by Johnson & Johnson, for emergency use in the fight against COVID-19. The Johnson & Johnson vaccine works somewhat differently than the Pfizer/BioNTech and Moderna vaccines, and though its protection level is reported to be measurably lower, it prevented against 100% of hospitalizations and deaths of those who did become infected with COVID-19.

In order to return to some semblance of normality, public health experts suggest that approximately 70% (herd protection level) of the population must be vaccinated against COVID-19. This will take an extraordinary effort, but without the public’s trust this level of vaccination will remain elusive.