Why does Omicron cause reinfection? Quick facts from WHO update

Global health agency shares emerging data-led theories on the new COVID variant’s transmissibility, infection risk and clinical severity

As suspected when the variant first emerged, Omicron may have a “growth advantage over other circulating variants”, said the WHO. (Representational image)

A year after the UK administered the world’s first COVID vaccine — on December 9, 2020 — the World Health Organization (WHO) has a hypothesis on why the Omicron variant may be able to overcome immunity mechanisms.

The latest variant of the SARS-CoV-2 virus is seen to pose a greater reinfection risk than earlier variants such as Beta and Delta. Citing preliminary analysis, the WHO said in its latest COVID update that this could be because the mutations present in Omicron “may reduce the neutralising activity of antibodies”. This could compromise the ability of natural immunity from a new infection.

Early studies ‘suggest’ that this property of Omicron may explain its rapid spread in South Africa, which has a highly immune population. While the African nation’s adult vaccination coverage is just 35%, its seroprevalence levels are at 60-80% due to past infections.

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However, the WHO said, more detailed studies are needed to confirm this. “Emerging data from South Africa suggest increased risk of reinfection with Omicron, but more data are needed to draw firmer conclusions,” said WHO Director-General Tedros Adhanom Ghebreyesus at a media briefing. This includes the variant’s ability “to infect or reinfect those who have been vaccinated, as well as to determine the severity of these breakthroughs or reinfections”.

How does Omicron fare in transmissibility?

As suspected when the variant first emerged, Omicron may have a “growth advantage over other circulating variants”, said the WHO, though researchers are yet to determine if this will translate into increased transmissibility. “We are now starting to see a consistent picture of rapid increase in transmission, although for now the exact rate of increase relative to other variants remains difficult to quantify,” it said.

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The WHO cited the European Centre for Disease Prevention and Control’s prediction that if 1% of COVID infections are due to Omicron now, it could quickly become the dominant variant in Europe. With a growth advantage of less than 120%, it could lead to less than 50% of new infections in Europe by January 1, 2022. With with a growth advantage of less 30%, it could reach the same level of infections by March 1, 2022.

While South Africa has seen an 82% spike in COVID hospital admissions in the week up to December 4, there is no data yet on how much of it is due to Omicron.

Does this mean vaccines and drugs may not be effective?

The WHO emphasised that interleukin-6 receptor blockers and corticosteroids are likely to be effective in severe infection management — there is no evidence currently to suggest otherwise.

To decide on the effectiveness of vaccines against Omicron, more data is required, particularly the use of additional/booster shots, it added. Researchers need to examine whether the mutations in the variant “may result in reduced protection from vaccine-derived immunity”.

However, vaccines are of paramount importance in COVID management, considering Delta is still the dominant variant worldwide, the WHO emphasised. “Accelerate vaccine coverage in the most at-risk populations, in all countries,” said Ghebreyesus. “Any complacency now will cost lives. We can save lives from Delta, right now. We can prevent Omicron becoming a global crisis, right now. And we can prevent other variants emerging, right now.”

How severe is an Omicron infection?

From late November, when Omicron was first identified, there has been a sharp rise in cases in South Africa (111%) and other nations in the neighbourhood — Eswatini (1,990%), Zimbabwe (1,361%), Mozambique (1,207%), Namibia (681%) and Lesotho (219%).

Apart from the fact that vaccination coverage is low in these countries, the infectiousness of Omicron could be a reason, said the WHO. However, the spike could also be due to increased testing with the onset of the new variant, it reasoned. Though the variant is seen to cause relatively milder infections with lower possibilities of fatalities, it is too early to judge its clinical severity, it added.

All this means the impact of Omicron on the overall pandemic cannot be determined now. While Delta is the dominant variant now, its virulence as against Omicron can be studied only after more data come in.

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