The Hindu Explains | Are there re-infection fears around COVID-19?

How long does immunity last? What are WHO and scientists saying about antibodies?

Updated - September 13, 2020 11:29 am IST

Published - September 13, 2020 12:51 am IST

The story so far: While the fear of COVID-19 re-infection has dogged discussion on the novel coronavirus , it was in late August that the first ‘confirmed’ case of re-infection was officially recorded. A 33-year-old Chinese male from Hong Kong reportedly caught his second infection during a trip to Europe, four-and-a-half months after he first tested positive for COVID-19. Post-testing, genomic sequencing made it clear that the first and second infection involved variants of the SARS-CoV-2 virus. This seemed to rule out viral shedding or continuing infection from the first time. Subsequently, a case of re-infection in Nevada, U.S., was also similarly revelatory, thanks to viral genome sequencing.

Are these isolated cases?

While there is no doubt this finding is significant, scientists are still debating whether this comprises an isolated few cases or portends a larger batch of infections as the world opens up and global travel begins again. Questions about waning immunity and the viability of a vaccine itself are still not settled either.

Also read | Bengaluru reports ‘first’ case of COVID-19 reinfection

What is immunity and how does it work?

What the discussion intrinsically hinges on is the ability of the human body to fight pathogens harmful to it, and whether in COVID-19 immunity wanes a few months after infection. The human body’s immunity acts in two forms — as innate, jumping to the task of protection immediately, and adaptive, meaning immunity acquired by the body in the process of surviving infection by pathogens, essentially over a period of time.

In a piece in The New York Times , Yale immunologists Akiko Iwasaki and Ruslan Medzhitov explain that the adaptive immune system consists of two types of white blood cells, called T and B cells, that detect molecular details specific to the virus and, based on that, mount a targeted response to it. “T cells detect and kill those infected cells. B cells make antibodies, a kind of protein that binds to the viral particles and blocks them from entering our cells; this prevents the replication of the virus and stops the infection in its tracks.”

Also read | Antibodies against coronavirus start to decrease in 2-3 months, study finds

T and B cells retain this memory and help the body fight the infection later. “Yet it is also the case that with other viruses the amount of antibodies in the blood peaks during an infection and drops after the infection has cleared, often within a few months: This is the fact that has some people worried about COVID-19, but it doesn’t mean what it might seem,” they add. “It’s a normal step in the usual course of an immune response. Nor does a waning antibody count mean waning immunity: The memory B cells that first produced those antibodies are still around, and standing ready to churn out new batches of antibodies on demand.”

Also read | Dead fragments of novel coronavirus led to false positives in recovered patients

What does it mean for the future?

Reacting to the Hong Kong case, Maria Van Kerkhove of the WHO said at a briefing: “There’s been more than 24 million cases reported to date… we need to look at something like this at a population level.” Researchers who studied the Hong Kong case themselves said in a publication in Clinical Infectious Diseases : “Our results suggest SARS-CoV-2 may continue to circulate among the human populations despite herd immunity due to natural infection or vaccination. Further studies of patients with re-infection will shed light on protective correlates important for vaccine design.” On its website, the WHO says it will continue to review the evidence on antibody responses to SARS-CoV-2.

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