The first confirmed case of a person being reinfected with the coronavirus was reported Monday in Hong Kong, a finding that could be a setback for vaccine development or just a minor wrinkle in the race to halt the spread of the virus.
The case involved a 33-year-old man who had mild symptoms the first time, and was asymptomatic the second time he tested positive 4½ months later.
The new study appears to bolster several studies conducted around the world that indicate the human body does not retain the antibodies that build up during coronavirus infections, raising questions about whether lasting immunity can be accomplished.
But Bay Area scientists said it is too early to sound the alarm because the double infection may represent the exception, not the rule.
"It's not surprising that we are going to see cases like this, but there is no evidence to date that this case represents a common occurrence," said John Swartzberg, an infectious disease specialist at the UC Berkeley School of Public Health. "Some people's immune response is not going to be sufficiently robust to prevent them from getting a second infection, but I think it's unlikely that this will be a common event."
The University of Hong Kong study tracked the reinfected man who was first diagnosed with COVID-19 March 26 after he complained of a cough, sore throat, fever and headaches that lasted three days. The second infection was discovered on Aug. 15 when the man took a saliva test after returning to Hong Kong from a trip to Spain.
He had no detectable symptoms the second time, but genomic sequencing showed he was infected with a virus that closely matched the strain circulating in Europe when he was there in July and August. It was a different strain from the one he had been infected with 142 days earlier, the researchers said.
Regular mutations create different lineages of the coronavirus, but most of them are identical in the way they affect people. Recent studies indicate a more contagious strain than the one first detected last year in Wuhan, China, circulated in Europe and has since spread around the world.
The report, published in the peer-reviewed journal Clinical Infectious Diseases, is a concern because antibodies are used to develop vaccines. If the antibody response lasts only a few months in some people, then a vaccine that relies on antibodies is not likely to generate immunity for any longer than that.
Reinfections could also skew efforts to reach herd immunity either with a vaccine or by letting SARS-CoV-2, the coronavirus that causes COVID-19, run its course until it can't find any more victims, as some people have advocated.
"First, it is unlikely that herd immunity can eliminate SARS-CoV-2, although it is possible that subsequent infections may be milder than the first infection," concluded the study. "Second, vaccines may not be able to provide lifelong protection against COVID-19."
The report comes on the heels of a study by King's College of London that determined that antibody levels peak three weeks after the onset of symptoms and then dramatically decline. A separate Chinese study also showed coronavirus antibodies taking a nosedive within three months. And other kinds of coronaviruses, like those that cause the common cold, regularly reinfect people. Antibodies for the influenza virus also wear off over time, requiring yearly vaccinations.
Robert Siegel, an infectious-disease specialist at Stanford University, said it would have major implications for society if reinfection with the coronavirus turns out to be common.
"For the individual, common infection means that frequent vaccination boosters may be necessary to prevent infection," Siegel said. "For society, this makes controlling the virus more difficult and reduces the hope that a high prevalence of immunity in a population may slow the spread of the virus."
But more study is needed before any conclusions are drawn, Siegel said. Researchers must first answer a number of questions. For example, is reinfection more common after mild or asymptomatic infections? Are reinfected patients contagious? Are specific strains of the coronavirus more likely to reinfect people?
Dr. Jay Levy, a specialist in immunology and virology at UCSF, said the new study appears to substantiate previous reports of people occasionally being infected by the coronavirus a second time, but he agreed with Siegel that the Hong Kong case could be an outlier.
"We would expect that some people could be reinfected perhaps because their initial infection was mild and they did not induce a strong memory response against the virus," Levy said, referring to one of the body's mechanisms for attacking viral intruders. "Or, a reinfection was detected before it was handled by the immune system."
The study's conclusions are being widely questioned online, not least for having a sample size of only one patient who was asymptomatic the second time.
Akiko Iwasaki, a professor of molecular, cellular and developmental biology at Yale University, tweeted that the study is actually a "textbook example" of how immunity is supposed to work.
"While immunity was not enough to block reinfection, it protected the person from disease," tweeted Iwasaki, who pointed out that antibodies were detected after the second infection, providing "a good example of how primary infection can prevent disease from subsequent infection."
And antibodies aren't the only measure of immunity, according to infectious disease specialists. The human immune system is made up of both B cells, which secrete antibodies, and T cells, which search out and destroy the cells containing pathogens.
Nadia Roan, an associate professor at UCSF and a T cell expert, found robust populations of T cells even after mild infections. The T cells persisted for months, and she was able to make them multiply in the lab. Other studies have found T cells that target the coronavirus even in patients with no antibodies, raising their potential value for a vaccine, Roan said.
Both T cells and B cells also exhibit a "memory" of infection, sometimes reappearing seemingly out of nowhere when a person is re-exposed to the virus. A goal of many vaccine developers is to figure out how to harness those unique capabilities.
Swartzberg said antibodies for SARS, a virus first reported in Asia in 2003, lasted between one and three years. But recovered patients are still showing T cell immunity today. Nevertheless, the Hong Kong study renews questions about how long immunity will last.
"I think for most of us, we are protected for months, at least, and probably longer than that," Swartzberg said. "But it's probably not going to be like measles, where once you get it you are protected for life."
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