Covid-19: recovered patients immune for months, but at high risk of infecting others – study

RIO DE JANEIRO, BRAZIL – Major research by UK health workers published in The Lancet suggests that most people remain immune for months, but may carry high levels of the virus in the nose and throat.

Most people who contract Covid-19 and recover are likely to be immune for several months thereafter, a study called SIREN involving over 20,000 health care workers in the UK and published in The Lancet has found.

The study concluded that past infection immune responses reduced the risk of contracting the virus again by 83% for at least 5 months.

“Over the course of the past year, reports of repeated infections with SARS-CoV-2 have undermined confidence in the immune system’s ability to maintain its defenses against the virus. The study’s interim results allay some of those fears,” explained lead researcher Susan Hopkins, senior medical advisor at Public Health England in London. “The data suggest that natural immunity may be as effective as vaccination, at least during the five-month period the study has covered so far.”

The data suggest that repeat infections are rare: they occurred in less than 1% of approximately 6,600 participants who had already been sick with Covid-19. But the researchers also found that people who are reinfected may carry high levels of the virus in the nose and throat, despite exhibiting no symptoms. “Such viral loads have been associated with a high risk of transmitting the virus to others,” Hopkins noted.

“Reinfection is quite unusual, so this is good news,” according to immunologist John Wherry of the University of Pennsylvania in Philadelphia. Although he cautions that “it doesn’t mean being free to walk around without a mask.”

SIREN is the largest study of coronavirus reinfection that systematically detects asymptomatic infections. Every 2 to 4 weeks, participants undergo blood tests to detect antibodies to SARS-CoV-2, as well as PCR tests to detect the virus itself.

During the 5 months, the team found 44 possible reinfections. In the group of 14,000 participants who had not previously been infected, 318 people tested positive for the virus.

“Some of the reinfections are still being assessed,” Hopkins explained. The 44 are considered “possible” reinfections and were classified on the basis of PCR testing combined with screening measures to reduce the risk of re-detecting the virus from the initial infection. So far, only 2 of these 44 cases have passed more stringent tests to be classified as “probable.” The study did not assess whether symptoms were better or worse during the second infection than during the first, but Hopkins notes that “only about 30% of people with possible reinfections reported any symptoms, compared with 78% of participants with first-time infections.”

For now, the team does not have enough data to determine who might be at greatest risk of reinfection. Immunologist George Kassiotis of the Francis Crick Institute in London notes that the study participants were mainly women, and mostly under the age of 60. “This group is unlikely to experience the most severe form of Covid-19, and may not be representative of the population as a whole.”

The researchers are still collecting data; they hope to get an idea of how long immunity lasts and to investigate the effects of a SARS-CoV-2 variant called B.1.1.7 that emerged in 2020 and has spread rapidly across the UK. “Although there is every reason to suspect that existing protection should cover new variants, it is possible that immune responses generated against one variant may be less effective against another,” Kassiotis concludes. That is still an open question.