Variants do not always evolve to become less virulent

Since transmission begins before symptoms set in and the disease becomes severe, its characteristic is decoupled from disease

In early February, World Health Organization technical lead on Covid-19, Dr Maria Van Kerkhove, cautioned that the pandemic is far from over and new variants will emerge and such variants could be more transmissible than the Omicron BA.2 variant. “The next variant of concern will be more fit, and what we mean by that is it will be more transmissible because it will have to overtake what is currently circulating. The big question is whether or not future variants will be more or less severe,” Dr Van Kerkhove said.

The only way the next variant can become even more transmissible than the Omicron variant is by exhibiting a far higher ability to evade neutralising antibodies. This would mean that full vaccinations (two doses) will be even less effective in preventing breakthrough infections. But so far, fully vaccinated people have been found to be less likely to suffer from severe disease requiring hospitalisation and even death. That is because it is the T cells and B cells that come into play to reduce the severity of the disease. “The memory T cells are extremely unlikely to prevent SARS-CoV-2 infections. That is just not what T cells generally do. They may reduce COVID-19 disease severity and prevent deaths,” Dr. Shane Crotty from La Jolla Institute for Immunology, La Jolla, California, had earlier told The Hindu.

“The variants are a wild card. We still don’t know everything about this virus, we still don’t know everything about the variants and the future trajectory of that,” she added.

While the next variant has to necessarily be more infectious than the Omicron variant, whether the variant will be more or less severe cannot be said with certainty. But it is important to remember that right from the very early stage of the pandemic, it became clear that transmission or virus spread begins even before symptoms can show up. That is what makes SARS-CoV-2 very different from the 2002 SARS virus or MERS virus. Since transmission begins even before symptoms set in and well before the disease becomes severe, the transmission characteristic is decoupled from disease. As a result, the natural evolution process selects variants not based on how they cause disease but how they can escape neutralising antibodies.

“Almost all [SARS-CoV-2] transmission happens while people have no or few symptoms, there is no particular reason for severity to play a role in evolutionary selection. NERVTAG [The New and Emerging Respiratory Virus Threats Advisory Group] thinks Omicron's mildness is likely pure chance and the next one is likely to be more severe again,” Dr William P. Hanage from Harvard T.H. Chan School of Public Health, Boston, tweeted.

The virus was novel and none in the world had any immunity in the beginning of the pandemic. But with millions being infected by the virus and millions being fully vaccinated, and some with a combination of natural infection and vaccination, the next variant has to necessarily exhibit higher immune escape to cause infection. This is the reason that the next variant will exhibit more immune escape than the Omicron variant.

Even though the Omicron variant caused a large number of infections in virus-naïve people and in those who have been previously infected and vaccinated, at the population level, disease severity has been far less severe compared with the Delta variant. But lower disease severity was seen more in people who have pre-existing immunity either from vaccination or previous infection.