Duration of protection against SARS-CoV-2 may be short

A study that monitored 10 healthy individuals for more than 35 years revealed that reinfection with the same seasonal coronavirus occurred frequently at 12 months after infection. This suggests that the duration of protection against SARS-CoV-2 reinfection too may not be long.

While it is generally known that protective immunity against the four species of common cold coronaviruses do not last for more than a year, a study that monitored 10 healthy individuals for more than 35 years revealed that reinfection with the same seasonal coronavirus occurred frequently at 12 months after infection. This suggests that the duration of protection against SARS-CoV-2 reinfection too may not be long, and reinfection, which is a common feature for all human coronaviruses, might be true for SARS-CoV-2 too.

The four common cold coronaviruses belong to two different genera and use different receptors found on different host cells that allow the virus to gain entry and infect the cells.

With over a dozen cases of reinfection with SARS-CoV-2, first reported from Hong Kong and then from the U.S and recently from India and other countries, as confirmed through genetic sequences, the key question centres around the duration of protection post an infection by SARS-CoV-2 virus. Insights gained from the longitudinal study published in Nature Medicine suggest reinfection with the same seasonal coronavirus occurred frequently around one year after previous infection. The short duration of protection against seasonal coronaviruses reinfection might be applicable for SARS-CoV-2 virus too.

Policy decisions

“We hypothesise that characteristics shared by these four seasonal coronaviruses, such as the duration of protective immunity, are representative of all human coronaviruses, including SARS-CoV-2,” the authors write. The findings of the study suggest that “caution may be needed” when coming up with policies by relying on long-term immunity either through natural infection or vaccination aimed at attaining herd immunity.

“I would not be at all surprised to see more reinfection cases [form SARS-CoV-2] to be reported in the future,” Prof. Akiko Iwasaki, an immunologist at Yale University, says in an email. “Since reinfection can occur, herd immunity by natural infection is unlikely to eliminate SARS-CoV-2. The only safe and effective way to achieve herd immunity is through vaccination,” Prof. Iwasaki tweeted.

How the study was conducted

For the study, the researchers used serum samples of adult males who were monitored at regular intervals since the 1980s to investigate how often seasonal coronavirus infections occurred during follow-up. The authors chose 10 healthy individuals from the cohort and serum samples collected from them every three months before 1989 and every six months afterwards were used for the current study; no samples were collected between 1997 and 2003.

Measuring antibodies

The researchers measured the antibodies to the nucleocapsid protein — an abundant coronavirus protein — for each seasonal coronavirus. An increase in antibodies was considered a new infection. While reinfections were seen in a few cases within six and nine months, they were more frequent at 12 months. No intermediate reduction in antibodies between infections were seen in the case of reinfections occurring as early as six months, but they did observe intermediate reductions between infections in the case of reinfections occurring more than six months.  

Since antibodies induced by coronavirus infections might have broad coronavirus-recognizing characteristics, they studied the complete nucleocapsid protein of SARS-CoV-2 to enable detection of broadly recognising antibodies. While two adults showed broadly recognising antibodies, and antibodies in one person remained present in the following years, no evidence of induction and maintenance of these antibodies were seen. “The sustained broadly recognising antibodies did not provide broad protection from subsequent infections,” they write.

The researchers were not able to identify strain variation among seasonal coronaviruses that could play a role in susceptibility to reinfection. Since there is only slightly varying circulating SARS-CoV-2 strains, increased susceptibility to reinfection by divergent virus strains of SARS-CoV-2 is most likely not the case, they note.

Published in The Hindu on September 19, 2020