Omicron: severe in virus-naive, unvaccinated people

Omicron has started to level the playing field, narrowing the gap between unvaccinated and vaccinated individuals

Updated - May 08, 2022 01:38 am IST

Published - May 07, 2022 07:10 pm IST

A health worker administers a dose of a COVID-19 vaccine at a special camp in Tamil Nadu’s Vellore.

A health worker administers a dose of a COVID-19 vaccine at a special camp in Tamil Nadu’s Vellore. | Photo Credit: The Hindu

As the SARS-CoV-2 virus continues to adapt to the human host, it has been transforming itself in ways that can escape immune protection. Omicron and its sub-lineages are ruling the world at this time, and it is important to know how well the vaccines are doing. For many reasons, real-world (field) experience is often different from the original efficacy studies conducted by vaccine manufacturers.

A&E visit

A recent study posted as a preprint in medRxiv, which is yet to be peer-reviewed, compared the real-world experience with Delta and Omicron, specifically addressing the risk of requiring a visit to the casualty or A&E (Accidents and Emergency, in the U.K.) among COVID-positive individuals of varying vaccination status. For the purpose of the study, visit to the casualty or A&E was used as an approximate marker of severity of disease, although an A&E visit can also be triggered for reasons other than COVID pneumonia The study by Daniel J. Grint and others looked at 2,37,430 people who got infected with Omicron, and 92,950 people with Delta in the U.K. during the month of December 2021.

They found, predictably, that Omicron-infected individuals were less likely to require an A&E visit than Delta. While the common explanation is that the severity of Omicron is lower because of widespread vaccination, the authors found that this difference was also apparent among unvaccinated individuals. In other words, compared to Delta, unvaccinated people with Omicron were less likely to require an A&E visit.

This could mean one of two things, or both. Omicron by itself could be less pathogenic, or unvaccinated individuals have acquired a substantial level of immunity by now through natural infection. The latter is more likely, because we know that unvaccinated people in Hong Kong suffered severely from Omicron. Natural infections were uncommon in Hong Kong because of their zero COVID strategy; this led to the absence of widespread natural immunity in the population. Thus, their suffering was not because the virus itself had become less lethal.

The lethality of the Omicron variant was highlighted by another large study. The recent study posted as a preprint in Research Square, which is yet to be peer-reviewed, by researchers from Harvard and Massachusetts General Hospital compared 1,30,000 COVID-19 patients during the Omicron and the Delta waves.

They found that contrary to popular belief, the severity of both these variants was almost the same, when measured in terms of hospitalisation and death rate. They discovered this after adjusting for confounding variables such as vaccination status, demography and comorbidities.

Immunity by vaccination

This indicates that the intrinsic severity of Omicron has not reduced, but rather our ability to counter the virus has improved — through immunity acquired by vaccination or natural infection.

The medRxiv study by Daniel Grint unearths more interesting findings pertaining to how well vaccines are doing with Omicron and Delta. In the case of the Omicron variant, the authors found that for most individuals, including healthy people over 70 years, it hardly mattered whether they were unvaccinated, vaccinated or boosted — they all had about the same risk of A&E visit.

However, elderly people who also had comorbidities were more likely to need an A&E visit if they were unvaccinated, implying that this subgroup of people stand to benefit the most from vaccination.

Marginal difference

Importantly, they also noted that with Omicron, the difference between those who were fully vaccinated with two doses and those who also received a booster shot was non-existent or marginal in most instances, with the exception of the same subgroup — elderly with comorbidities.

However, in the case of the Delta variant, unvaccinated people in all adult age groups were clearly more likely to need an A&E visit compared with those who were vaccinated.

They also report that compared to Omicron, the proportion of vaccinated and boosted individuals was considerably lower among the Delta-infected subgroup, implying that vaccine protection was higher against Delta.

The findings indicate that Omicron has started to level the playing field, narrowing the gap between unvaccinated and vaccinated individuals, a trend that was not observed with Delta.

This does not mean that vaccines are ineffective, as the overall risk of A&E visit is still greater among the unvaccinated. The study helps to categorise those who are more or less likely to experience a difference, by virtue of being vaccinated or boosted.

Compared to 2020, at least three major changes have occurred in the pandemic narrative, as we gather more evidence. First, while vaccines continue to protect against severe disease, their ability to stop infection or transmission is not good enough. Second, reinfections are common. This means that people who were vaccinated or unvaccinated could continue to repeatedly pick up infections if they got exposed to a large viral aerosol load. Finally, the risk of infection is lower in the initial few months following a vaccine dose or natural infection, but this early protection from infection rapidly wanes afterwards.

(Rajeev Jayadevan is co-Chairman, National IMA COVID Task Force)

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