April 15, 2020

Editor's note: This popular story from the Daily Briefing's archives was republished on June 3, 2020.

As state and federal officials consider whether it's safe to lift social distancing measures, one key unanswered question is whether exposure to the virus could protect people from future infection.

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Is anyone immune to the new coronavirus?

In a New York Times opinion piece, Marc Lipsitch, an epidemiologist and infectious disease specialist at Harvard T.H. Chan School of Public Health, writes that for some viruses, such as measles, when a person becomes infected, they recover and become immune to the virus for life.

In the case of the new coronavirus, known as SARS-CoV-2, little is known at this point about how the virus affects immunity. In fact, Lipsitch notes that research on immunity to coronaviruses in general—which includes a large group of viruses that cause diseases such as MERS, SARS, common colds, and pneumonia—is limited. But it could help to inform officials' decisions on lifting social distancing measures. Here's what the research says.

The research

According to Lipsitch, research on coronavirus immunity has always reared "complicated" results.

Most of the existing studies on coronaviruses center on seasonal coronaviruses that cause infections such as the common cold and pneumonia. In those studies, researchers infected human participants with a seasonal coronavirus over multiple years to determine whether an initial infection resulted in immunity.

In one such study conducted in the 1970s, researchers exposed a number of participants to a seasonal coronavirus. The researchers then selected 18 participants who had developed colds after being infected with the virus. They exposed six of those subjects to the same viral strain and the remaining 12 subjects to a slightly different strain of coronavirus one year later. None of the six participants who were re-exposed to the same viral strain became infected upon their second exposure, likely because their bodies built up immunity to the virus after their first exposure. However, the other 12 participants who were exposed to a slightly different strain of coronavirus developed only partial immunities against the new virus.

In a similar study published in 1990, 10 of 15 volunteers who were inoculated with a coronavirus were infected upon exposure. When 14 of the participants were exposed to the same virus one year later, their bodies displayed more mild symptoms than when they were first infected. According to Lipsitch, the findings imply that the participants had developed an immunity that protected them against a more severe infection, though they still were infected.

Lipsitch notes that similar studies haven't been conducted to examine immunity to SARS and MERS, which are caused by coronaviruses that are similar to the new SARS-CoV-2, which causes Covid-19. However, he writes that measurements of antibodies in the blood of people who have recovered from MERS and SARS infections suggest patients may gain some degree of immunity against those viruses for a limited amount of time: about two years for SARS and about three years for MERS. However, Lipsitch notes that the research also showed that "the neutralizing ability of these antibodies—a measure of how well they inhibit virus replication—was already declining during the study periods."

According to Lipsitch, a recent peer-reviewed study from Erasmus University in the Netherlands similarly found that 12 patients developed antibodies after recovering from an infection with SARS-CoV-2.

Research on coronavirus immunity still is inconclusive, experts say

While those "studies form the basis for an educated guess at what might happen with Covid-19 patients," health experts say there is not enough data to distinctly determine whether people who have recovered from the new coronavirus have any degree of immunity from the infection later on, Lipsitch writes.

According to Lipsitch, some experts have estimated that "[a]fter being infected with SARS-CoV-2, most individuals will have an immune response, some better than others. That response, it may be assumed, will offer some protection over the medium term—at least a year—and then its effectiveness might decline."

However, data also suggests there are patients who may not build up any immunity to the new coronavirus—and therefore might be at risk of reinfection. Lipsitch notes that South Korea's Centers for Disease Control and Prevention has reported that 91 patients who had been infected with the new coronavirus and later tested negative for the virus, then later retested positive for the virus. While the report raises the possibility of reinfection, it's also possible that the "patients had a false negative test in the middle of an ongoing infection, or that the infection had temporarily subsided and then re-emerged," Lipsitch writes.

Further, one study on the new coronavirus indicates that not every patient has the same immune response to infection, meaning some patients may be more susceptible to reinfection than others.

More research could facilitate better-informed responses

Lipsitch writes that the current uncertainties surrounding immunity to the new coronavirus likely will become clearer when research, as well as tests that detect infection and antibodies from the new coronavirus, become more widespread. "We need to design studies where individuals with known Covid-19 infection and without infection are followed over time to assess whether the first group is protected, or partially protected, against Covid-19 infection compared to the second group," Lipsitch writes.

And Lipsitch notes that it's important for researchers to get answers about immunity to the new coronavirus quickly, as officials until then must make "decisions with great consequences" without "definitive data." He writes, "[G]etting a handle on this fast is extremely important: not only to estimate the extent of herd immunity, but also to figure out whether some people can re-enter society safely, without becoming infected again or serving as a vector, and spreading the virus to others" (Lipsitch, New York Times, 4/13; Aubrey, "Shots," NPR, 4/13).

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