COVID-19 Vaccines 101: FAQ

What is the interval between doses?

For the Moderna vaccines, the two doses are given 4  to 6 weeks apart. For Pfizer's vaccine, the two doses are given 3 to 6 weeks apart. The U.S. government and the manufacturers have partnered to make sure there are enough doses available for everyone to get two. 

What happens if you don't take the second dose?

Protection is assumed to be less. In data that Moderna submitted to the FDA before its Dec. 17 review for its request for emergency use authorization, for instance, its analysis suggested that the first dose provides protection from getting COVID-19, but the data did not allow for a “firm conclusion," the FDA says. Both the Pfizer and Moderna vaccines are believed to be around 50% effective after just one dose. 

After the required doses, how long until it takes effect and provides protection?

That happens “about a week after the second dose," says Naor Bar-Zeev, PhD, an associate professor of international health and vaccinology and deputy director of the International Vaccine Access Center at Johns Hopkins Bloomberg School of Public Health. Once the second dose kicks in, both the Pfizer and Moderna vaccines have shown in studies to be about 95% effective. The J&J effectiveness data was based on 28 days after the inoculation. 

How well do the vaccines work?

Overall, the Pfizer and Moderna vaccines are about 95% effective.Overall, the J&J vaccine was found 72% effective in preventing moderate-to-severe COVID-19 in U.S. studies.   

How long does the protection last?

Because the vaccines are new, this is not yet known for sure. Based on other viruses that are similar to the coronavirus that causes COVID-19, the COVID-19 vaccines that are shown to be highly effective might protect people for a few years, says Paul Offit, MD, director of the Vaccine Education Center and a professor of pediatrics at Children's Hospital of Philadelphia. That's an educated guess based on his expertise and known facts about the virus that causes COVID-19.

What about side effects?

People should expect to have some side effects, similar to what some people report after getting a flu vaccine, according to experts meeting recently with the CDC. These experts said to expect temporary side effects such as soreness in your arm where you got the shot, fatigue, body aches, and perhaps a fever.

At least two people getting the Pfizer vaccine in the U.K. had severe allergic reactions, promoting the British government to tell those prone to anaphylaxis-like reactions not to take it.

As for serious side effects, it's too soon to know about rare side effects, says Bar-Zeev. That is because phase III trials do not supply enough information about rare side effects. That will become evident, if it occurs, as more people are vaccinated. Even so, Bar-Zeev says it's not a reason to avoid the vaccine.

As millions of doses are distributed, if there are very rare side effects, they are expected to show up in a very short time frame, Offit says.

After the FDA's emergency authorization (EUA) is granted, are the vaccines still tracked?

Yes. The FDA expects the manufacturers to continue their clinical trials to find out more about how safe and effective they are, and pursue full FDA approval or licensure. The EUA, which is different from FDA approval, is based on the FDA's evaluation of available evidence, assessing risks and benefits. It issues the EUA if the benefit-risk balance is favorable.

Do the COVID vaccines not only keep the person from getting sick, but also from spreading the virus if exposed?

That is not yet known. As more data and monitoring are done, experts will be able to find out if a vaccinated person, if exposed to the virus, can still spread it even if they don't get the disease themselves, says Bar-Zeev of Johns Hopkins.

Are the vaccines free?

Yes, for patients, but the health care providers will bill insurance companies, Medicaid and Medicare, or tap federal funds for the uninsured. In one estimate, the cost per dose was $37 for Moderna's vaccine, $20 for Pfizer's, and $10 for J&J.

Will it be possible to choose which vaccine you prefer?

In general, it does not matter, since once a vaccine gets the FDA's emergency use authorization (EUA), they all work. And even as more vaccines become authorized and available, you may have only one choice.

If a vaccine needs two doses, can you switch to another vaccine for the second one?

No. Experts advise staying with the same vaccine for both. That's true even for the Moderna and Pfizer vaccines, which use the same general approach yet are different.

How much of the population is likely to get vaccinated?

It's not possible to say, although surveys have cited a problem with “'vaccine hesitancy," even among health care workers. But that may change. "I think this will be a desired vaccine," Offit says.

How much of the population needs to be vaccinated for so-called herd immunity?

"Herd protection is not a goal of the initial rounds of vaccine deployment," Bar-Zeev says. "Only once population-wide vaccination is a reality would herd protection be even considered."

After I get vaccinated, do I still have to wear a mask?

Yes. Even after vaccination increases, say Bar-Zeev and other experts, preventive behaviors will still be needed. "The ability to reduce transmission will require not just high vaccine uptake, but ongoing social distancing and masks," he says. And herd protection may require high rates of vaccination in groups that are themselves at low risk, he says, “so [that] raises ethical questions."

When can we expect to get vaccinated?

On March 2, President Joe Biden announced that he had ramped up production, vowing there will be enough vaccine doses so every U.S. adult could be immunized by May.