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Yes, people who’ve had Covid-19 can still benefit from the vaccine

Immunologists explain why.

Pharmacist Khanh Pham calls a person over before injecting them with the Covid-19 vaccine at the Santa Clara County Fairgrounds in San Jose, California.
Randy Vazquez/Bay Area News Group/Anadolu Agency via Getty Images

In the United States, there have been 17 million reported cases of Covid-19 — about 5 percent of the population. Millions more have been infected and either did not get tested or didn’t have symptoms to begin with.

Vaccines are slowly rolling out — first to front-line health care and high-risk people, and next, likely to other essential workers, followed by people ages 65 and older and those with preexisting conditions.

But it’s reasonable to ask, given that there are well over 100 million people in these groups and not nearly enough vaccine supply for them right away: Should people who’ve already had Covid-19 be vaccinated too?

After all, the body mounts an immune response to the virus during an infection. In lab studies, scientists find that most people who contract the virus develop neutralizing antibodies to it. These antibodies are the immune system proteins that bind to viruses and render them harmless.

So a person who has had the virus likely has developed some level of immunity. Still, immunologists and vaccine experts say these people can — and perhaps should — get vaccinated anyway, should a vaccine become available to them.

“If I, personally, had Covid, I would still like to be vaccinated,” Alexander Sette, an immunologist at the La Jolla Institute for Immunology, says.

He and other immunology and vaccine experts can explain why.

Why people who have had Covid-19 should still be vaccinated

The biggest reason everyone — whether or not they had Covid-19 in the past — should be vaccinated is because different immune systems have responded very differently to the virus.

In general, Sette says, a body will mount a durable immune response. “In fact, we’ve seen it persist up to eight months,” he says. But this is only the case for 90 percent of people. “For 10 percent of people, they don’t seem to have a good immune response eight months out.”

There have been some documented cases of reinfection, which suggests that in some people, the immunity that comes from a first infection is either weak or wanes over time (scientists still don’t know how common reinfection is). Generally, scientists have told us, the worse the first infection, the stronger the immune response will be.

The thing is, “we have no way to tell,” he says, at least easily and feasibly, if a formerly infected person is in the 90 percent or in the 10 percent. Not taking precautions like masking and social distancing, or avoiding a vaccine after an infection, he says, is like “driving a car where you’re 90 percent sure the car has brakes.”

This has been the story of the whole pandemic: The human body’s response to the SARS-CoV-2 virus that causes Covid-19 has been extremely variable. Some people don’t get symptoms at all. Some people end up dying of the infection. “Some develop very high levels of neutralizing antibodies and are likely in no need of vaccines, while others develop undetectable levels of neutralizing antibodies,” Yale immunobiology researcher Akiko Iwasaki explains in an email.

So, quite simply: A vaccine helps level out that variability. People whose immune systems didn’t produce a robust response can catch up to those who did. According to Sette, there’s less variability in people’s immune responses to the Covid-19 vaccines than there is to natural infection.

And, again, individuals who have had Covid-19 can’t really gauge their own levels of immunity. Yes, a person could potentially get an antibody test, to see if there are any in their bloodstream.

But the immune system has many, many other components, from memory B cells that can be activated to produce antibodies in the future, to T cells, which kill and destroy infected cells. These are not commonly tested for. And even if an individual could be tested for all the components of an immune response to SARS-CoV-2, it’s still hard to know what it all means for a second infection. Scientists still don’t understand the “correlate of protection” for Covid-19. That is: What is the precise right mix of immune system cells for a particular person that would prevent them from getting infected again?

Sette says it’s a “reasonable argument” to suggest that maybe people who have had Covid-19 shouldn’t be prioritized to get the vaccine, should the stock of it remain limited over the long term. But in practice, that argument could become problematic.

For one: “It’s too difficult to operationalize pre-vaccination testing,” Peter Hotez, vaccine expert and the dean of the National School of Tropical Medicine at Baylor College, says in an email. It would be tedious work to determine who might be immune to SARS-CoV-2 before vaccination. It could be tedious work to determine who has been infected prior to vaccination as well.

Plus, again, the information resulting from such tests may not be all that useful in determining a person’s lasting immunity.

The vaccines are safe — regardless of whether you have been infected

To sum up: “To be safe, I recommend getting the vaccine, even after you recover from COVID, when the vaccines become sufficiently available,” Iwasaki says.

It’s still a little unclear what the vaccine would add on top of a person’s natural immune response to the infection. Would a person who made a weak immune response to a natural infection make a stronger immune response to a vaccine? It’s possible.

“During a natural exposure to SARS-CoV-2, there are multiple factors that interfere with a robust immune response,” Iwasaki says. “The exposure dose may be too little. The virus interferes with our immune system (both innate and adaptive) to block proper antibody induction.”

On the other hand, she says, “vaccines are formulated to provide just the right dose” of the viral protein, and there’s no live virus to interfere with out immune systems. “There appears to be a much more uniform and higher level of antibodies generated with a vaccine,” she says.

For now, the Centers for Disease Control and Prevention isn’t making an official recommendation on whether people who have had Covid-19 should be vaccinated; it’s waiting on the input of the Advisory Committee on Immunization Practices, a group of medical and public health advisers who make recommendations on how to vaccinate the public. Though we know from clinical trial data, and the Food and Drug Administration’s review of it, that the Pfizer/BioNTech and Moderna vaccines appear to be very safe for the general public.

But have they been proven safe, in particular, for people who have already been infected? There may not yet be enough data to say that definitively. “I think the answer is probably yes, but we won’t know for sure until the numbers are larger,” University of Washington immunologist and physician Helen Y. Chu writes in an email. “For most of the phase 3 trials, there was no screening for pre-existing antibody.”

Keep in mind that lack of safety data for this particular group doesn’t mean it’s unsafe. As Chu explained, there just needs to be more data.

Both the Moderna and Pfizer/BioNTech trials did include a small number of people who had already been infected, though. On Thursday, Moderna vaccine scientist Jacqueline Miller told an FDA advisory committee that the company is “anticipating data in the coming weeks” on how and whether the company’s vaccine boosts the immune systems of people who had been previously infected.

Also unclear from the current data: whether the vaccine truly does give people who mounted a weak immune response to a natural infection — that is, people who either did not produce a lot of immune system cells to fight the virus or whose immune system cells to fight the virus have declined over time — an immunological boost.

“This is not answered yet, but I would say that it probably does not hurt,” Chu says. “Antibody wanes over time, and it is likely that the vaccine will boost your pre-existing antibody titers.”

That said, per the current trial data, both vaccines are around 95 percent effective at preventing Covid-19. Sette says that high a level of efficacy is probably indicative that the vaccine can produce a robust immune response in a broad majority of people.

The language of vaccine science is really tricky. To say a vaccine protects against disease is not the same as saying the vaccine makes a person completely immune (or unable to spread the virus). Perhaps some people still get infected but clear the infection before symptoms show up. Scientists will need more data on this fine distinction. That said, “it would be hard to fathom that the vaccine gives you a 95 percent protection without inducing an immune response,” Sette says.

There’s a lot about decision-making during the pandemic that’s been very difficult. Deciding whether, and how, to visit friends and loved ones leads to a tedious risk-benefit analysis. Luckily, with the vaccines, this decision matrix is a lot simpler: Even if you have had Covid-19 in the past, a vaccine may help prevent future infections. Yes, more data is needed to be absolutely definitive on this. But for now, this is all pretty encouraging.

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