DH Deciphers | What causes the rise and fall of Covid waves?

India’s first Covid-19 wave peaked last September and since then even though vaccines weren’t rolled out and stringent lockdown measures like those in March-May of 2020 weren’t imposed, cases started to drop over time. About six months later, in March this year, cases started to surge in what is now known as the second Covid-19 wave. Now, as the second wave tapers off, here's what we know about the rise and fall of Covid waves and why the second wave may be slowing down:

What could be the reason behind the second wave?

According to a study by the Council of Scientific and Industrial Research, about 20% of people who were seropositive (those who had Covid antibodies) lost their antibodies in the six months after the first survey. Researchers behind the study say that this could in part explain why India saw the second wave six months after the first. Serological surveys examine the number of people who have antibodies that work against a disease. About 75% of those who were seropositive did not report any symptoms.

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Why and how people lose virus-neutralising antibodies is an observed biological phenomenon but remains unexplained.

What causes a dip and rise in Covid-19 waves?

In the study, researchers argue that based on falling test positivity rates, the transmission of Covid-19 decreased in regions that displayed high seropositivity. Emphasis on Covid-appropriate behaviour such as social-distancing and masking, too, spurred a drop in cases along with a high prevalence of antibodies in the population. About 10.14% of people in the study were seropositive, which suggests that there was a large number of those who recovered from Covid-19 by September 2020, especially among those who work in high-contact occupations, leading to a drop in new infections.

Data collected by the council shows that 20% of seropositive individuals lacked meaningful neutralisation activity from the antibodies after five to six months. These 20% of people are fresh candidates for reinfection after this period, while the rest may have antibodies that are stable for longer. This drop in antibody prevalence over time is supported by studies conducted in other parts of the world, bringing about a strong correlation between a drop in Covid cases and antibody prevalence over time.

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"The rise or fall of an epidemic is roughly proportional to the prevalence of non-immune population that leads to a rise and a high proportion of immune population that slows down the spread," says Dr T Jacob John, eminent virologist and former head of the ICMR's Centre for Advanced Research in Virology.

So will the next Covid wave start in six months?

It cannot be guaranteed that the next wave will arrive in precisely six months as there are many factors that cause Covid-19 waves. "Once a wave has come down naturally, another wave will require virus variants that are more infectious than those in the second wave. In other words, antibody studies cannot predict waves," says Dr John. He adds that protection from reinfection lasts longer than just six months.

This cements the argument that though antibody-based serological surveys may act as a rough guide to when the next wave might come, they cannot be fully relied upon as virus variants are not accounted for in these studies.