Frequently Asked Questions about COVID-19 Vaccines


Vaccines are complicated and sometimes controversial. Our team of medical professionals and public health experts composed answer common questions about COVID-19 vaccines. This includes the most promising vaccines being developed, efficacy rates (how successful they are), and safety from an unbiased, research-based perspective.

For more detailed research analysis and clinical trial updates, see

For all inDemic Perspectives regarding vaccines, see

Frequently asked questions

How do the vaccines work?

For us to develop immunity against a virus, we must be exposed to something similar to that virus. The SARS-CoV-2 virus consists of many proteins, but it is dependent on its spike protein to infect human cells. Most of the vaccines under development deliver spike protein into our cells, so that we mount an immune response against that spike protein. This immune response protects us against any future exposure to spike protein, which would only occur if we’re exposed to the SARS-CoV-2 virus, or if we get a booster shot of a spike protein-based vaccine

What makes a vaccine more/less effective? For example, what does it mean that Pfizer/BioNTech’s vaccine is 95% effective and Moderna’s vaccine is 94% effective?

That’s a good question. I’ll try to answer it in a couple ways.

In terms of the numbers, the Moderna and Pfizer vaccine protocols use a simple formula to calculate the vaccine efficacy, 1 minus the (COVID-19 rate in the vaccine group divided by the COVID-19 rate in the placebo group), expressed as a percentage. If the vaccine and placebo group developed COVID-19 at the same rate, 50 cases out of 15000 volunteers in both groups, then based on the formula, the vaccine efficacy is 0%. If, on the other hand, no COVID-19 cases are detected in the vaccine group, and all the COVID-19 cases are detected in the placebo group, then based on the formula, the vaccine efficacy is 100%. So, in a clinical trial or across a population, a more effective vaccine is one that reduces the natural rate of COVID-19, compared to placebo.

In terms of vaccine design, both Moderna and Pfizer’s mRNA vaccines encode the full-length, prefusion spike protein, encapsulated within a lipid nanoparticle. The spike protein is what allows the SARS-CoV-2 virus to attach to and infect human cells. It seems like it’s a good “target” because both vaccine candidates are showing ~95% efficacy.

In the future, we may be able to say that an “effective” vaccine is one that produces a strong antibody response or a strong type 1 helper T cell response, and that’s certainly of interest to scientists looking to develop the next generation of vaccines. These immunological correlates might also be useful in giving us insight into how and why a small number of vaccinated participants still develop COVID-19.

I want the vaccine. When can I take it?

For the first couple of months after FDA authorization of the mRNA vaccines, they’ll be in short supply. Millions of doses are available now, but over half a billion doses are needed to vaccinate everyone in the USA. Most states will vaccinate healthcare workers and long term care facility residents first.

For updated vaccine timelines, see

For detailed information on how different groups, countries, and states decide vaccine prioritization, see

I don’t want the vaccine. Can I refuse to take it?

Yes, most likely. The US government is unlikely to mandate vaccination. However, it’s possible that certain employers will mandate vaccination; for example, a hospital may mandate its physicians, nurses, and other clinical staff to be vaccinated as a condition of employment. Exemptions for medical, religious, or personal reasons may be allowed, as this is common practice for vaccines against other viral illnesses.

Who will pay for the vaccines?

The US government has paid for vaccines from Moderna and Pfizer. The vaccines will be available to everyone in the US for free.

Is it safe to take both vaccines consecutively? What happens in general when someone gets two doses of the same/similar vaccine?

That’s interesting.

First, I doubt that anyone will be able to take both vaccines, at least in 2020 or 2021. We’re likely to have a shortage of vaccines, and they’ll be prioritized for those with high-risk occupations (e.g. healthcare workers) or those with high-risk medical conditions (e.g. heart disease). Plus, the government is actively involved in manufacturing, distribution, and payment of the initial wave of COVID-19 vaccinations. The likelihood of a wealthy and paranoid individual buying up thousands of doses of vaccines for him or herself is not likely.

Second, if we allow ourselves to speculate, even if someone took both the Pfizer and Moderna vaccine, it wouldn’t necessarily do anything. More is unlikely to be better. Like all phase 1 trials, the Pfizer and Moderna phase 1 vaccine trials tested several dose levels. The dose levels they carried forward for testing in their phase 3 trials had the best balance of safety (e.g. reactogenicity) and efficacy (at least in terms of antibody response, etc.).

I’ve received the first dose of the COVID-19 vaccine. When should I receive the second dose of the COVID-19 vaccine?

For the Pfizer/BioNTech COVID-19 mRNA vaccine (BNT162b2), the second dose should be given 3 weeks after the first dose. In Pfizer/BioNTech’s phase 3 clinical trial, 98% of participants received two doses of the vaccine at a 3-week interval. Ideally, two doses of BNT162b2 should be given 21 to 28 days apart. 
For the Moderna COVID-19 vaccine (mRNA-1273), the second dose should be given 4 weeks after the first dose. In Moderna’s phase 3 clinical trial, 92% of participants received two doses of the vaccine at a 4-week interval. In addition, Moderna’s phase 3 clinical trial’s per-protocol analysis included all participants who received two doses of the vaccine 21 to 42 days apart. Ideally, two doses of mRNA-1273 should be given 21-42 days apart. 

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