I’ve already had COVID-19. Do I need one vaccine shot or two?

This is one of a series of articles in which reporters from WHYY’s Health Desk Help Desk answer questions about vaccines and COVID-19 submitted by you, our audience.

Since the beginning of the COVID-19 pandemic, public health experts, scientists, and the rest of us have wanted to know whether contracting the coronavirus once protects people from getting infected by it again.

A year in, scientists are closer to a more definitive answer. As vaccines roll out, emerging data could also provide answers about whether vaccines are necessary for people who have recovered from COVID-19.

The University of Pennsylvania’s Penn Institute of Immunology has been studying immune responses to the vaccines among people previously infected with the virus and comparing them to those of people who have never contracted it.

Those who previously had COVID-19 had a strong immune response after just one dose of the Moderna or Pfizer vaccines, the researchers found. Those who have never had the virus were less protected, however.

Though the researchers are not making any clinical or policy recommendations, they believe their findings will help scientists understand more about the virus, the vaccines, and how people respond to them.

Do I need only one dose of the vaccine if I’ve had COVID?

The COVID-19 vaccines induce very potent immune responses and antibodies, the researchers found, as well as inducing B and T cells that are important for protection from viruses.

Immune responses to vaccines and infections result in the production of antibodies that provide rapid immunity, as well as the creation of memory B cells that help with long-term immunity. Penn researchers looked at how memory B cell responses differed after vaccination in those who previously had COVID and those who had not.

For people who’d already had mild or asymptomatic COVID-19, one dose of vaccine was enough to induce a maximal immune response, based on both strong antibody and memory B cell responses. That’s likely because their previous infection by the virus caused an immune response. Adding a second dose did not offer much more protection for the people in the Penn study.

“What we know from the literature is that people with mild or asymptomatic infection often have lower levels of antibody than those who had severe infection. So it looks like in this group of individuals that had mild infection, that one dose of vaccination really does boost the immune response, especially boosting their neutralizing antibodies to variant viruses,” said John Wherry, director of the Penn Institute of Immunology.

If getting COVID produces antibodies, why get vaccinated at all?

Epidemiological data from countries like Israel, Denmark, and the U.K. suggest that people who get COVID-19 are well protected from severe illness if they contract it a second time.

There are some concerns, however. Wherry said that even if a person gets a mild or asymptomatic second infection, that person can still shed the virus to others. There’s also some concern that contracting the original strain of the virus might not offer protection from one of the variants.

“The idea of one vaccine dose in these folks is like boosting something that the natural infection generated in them, providing, we hope, a little bit better protection,” Wherry said.

How long do antibodies last?

The simple answer is scientists don’t know.

New data from vaccine manufacturers Moderna and Pfizer show there are high levels of antibodies about six months after vaccination. But scientists still don’t know whether those could last a year, three years, or longer.

“I think, very likely, those antibodies from vaccination are going to be present for a while. And if we’re talking about boosters in the near term, we’ll probably talk about boosters because of the variant viruses — more so than because antibodies have declined,” Wherry said.

Do I need two doses if I’ve never had COVID-19?

The Penn study found that the research participants who were never infected with COVID-19 did not have a full immune response until after receiving the second dose.

Wherry said it’s very important that people who have never had COVID get both doses of the Pfizer or Moderna vaccines, to get optimal high levels of antibodies.

“Both doses of the vaccine are necessary to get a neutralizing antibody, really the highest quality antibody, not just against the [original strain of the virus], but also against some of the variants that are currently causing a lot of us some concern,” he said. “If you only get one dose, levels of those antibodies are very low, undetectable, actually, in some people.”

Data from clinical trials shows there is about 80% protection after one dose of the Pfizer and Moderna vaccines. But everybody in those trials received second doses three or four weeks later. So scientists don’t know how long the protection after one dose lasts.

“We know that the first dose does not give you the full immune response that you get from having two doses close together,” Wherry said. “We see a little bit of protection, and that’s a really good thing. But we do not know how long it lasts. We don’t know that it’s going to be efficacious for a long period of time. So I think it’s a big risk to not get the second dose of the vaccine.”

What if I had COVID and didn’t know it? Is it safe to get the second dose?

The Penn study did not show any evidence that a second dose could cause harm among those who have previously had COVID-19.

In general, though, health experts and scientists look at risk versus benefit.

“Right now, people get side effects, side effects make you feel kind of sick … they’re actually telling you that your response is working. Side effects are different from adverse events. I would not classify the side effects of having a day of chills as being a big risk,” Wherry said.

“The adverse events, the very, very rare complications of vaccination would be what you might want to think about in the risk/benefit equation. They’re extremely rare, 1 in 100,000 to 1 in a million. And so far, these adverse events that we’ve heard about in Europe from the [two-dose] AstraZeneca vaccine or the [one-dose] Johnson & Johnson seem to be occurring mostly in people who’ve never had COVID before. So I think the question is, if you’re asymptomatic, you don’t know it, and you get both doses of vaccine, you’re perfectly fine.”

What more do scientists want to know?

Wherry said scientists want to learn about the durability of the vaccines, and how long immune responses remain stable and strong. They also want to understand more about the different features of the immune response, and which ones are most important, he said.

“If we want to track long term, and make some predictions about when you might need a booster, we want to understand the individual features of the immune response, so we can start to predict when someone might be susceptible or need a booster or need to pay attention to their social activities,” Wherry said.

Scientists also don’t fully understand how people might respond differently to the vaccine: how the vaccines behave differently in older people; how the vaccines behave in people with underlying conditions like cancer or an autoimmune disease.

What do we know about the role of genetics and other factors in side effects?

Thinking about this is just in its infancy, Wherry said.

So far, the Centers for Disease Control and Prevention has been asking people to self-report their side effects via an online form known as V-Safe. Some scientists say those who reported more severe side effects or systemic side effects have slightly better immune responses. That doesn’t mean someone with mild side effects doesn’t develop a good immune response to COVID-19. But someone with a fever and chills might have high levels of antibodies.

Wherry said there is interest in looking at whether underlying genetics might have something to do with that, whether it has to do with receiving a particular vaccine, or whether there’s a relationship with age, because older people often have less severe side effects.

“I think there are many possible contributors here, and many of us are looking at this carefully,” Wherry said. “It’s a bit challenging, because we have to accurately quantify those side effects, and for the most part, they’re self-reported. And so what might feel like a horrible raging headache to one person may be an annoyance to another person. And so on. So we have to get an accurate description of the side effects and pain.”