Waning immunity may require wider boosters

Large section of those vaccinated got their last dose more than 6 months ago, leading to protection tapering.

New Delhi: A little over a third (36%) of all Indians above the age of 45 received their second dose of the Covid-19 vaccine at least six months ago – a time threshold that many studies say causes significant waning of immunity – pointing to the need for the country to make available booster shots to this population cohort, and progressively to everyone over 18 years of age. Currently, only those over 60 years are eligible for boosters in India.

Booster doses for senior citizens and frontline workers were opened on January 10. (Vijay Bate/HT Photo)

Several global studies, both in laboratory setting as well as those that looked at real-world data, have shown that vaccine effectiveness against severe Covid-19 starts decreasing after a certain time period, widely believed by experts to be around four to six months after people were administered the second dose of the vaccine.

Indeed, one theory for the surge in infections, including those caused by a second Omicron variant-driven wave in some Western countries, is waning immunity.

Of India’s estimated 345 million adults above the age of 45 years, according to population projections for 2021 by the Census of India, 122.8 million had received both shots of the vaccine by September 20, 2021. This means that more than a third had received both shots.

In the weeks that followed, this number saw a massive increase as vaccination in this category picked up pace. By October 20, around 43% of Indians above 45 years were doubly vaccinated, while a little over half (53%) of this population group were fully vaccinated four months ago, as on November 20, 2021.

These numbers mean that in around eight weeks’ time, more than half of all Indians above 45 years of age will have crossed the six-month threshold of their second shot.

Among the studies on waning immunity are those that looked at the Oxford-AstraZeneca vaccine, which is manufactured in India under the brand name Covishield, and is currently responsible for around nine out of 10 shots administered in the country.

Since the contraction of India’s third wave, new Covid-19 cases in the country right now are the lowest since the early days of the outbreak more than 22 months ago.

Of India’s total eligible population, 80.9% have received two shots of the vaccine, while another 14.5% have been administered a single shot.

Experts said that while the third Omicron wave may have bought India some time in terms of natural immunity among the masses, the sooner the issue of wider booster coverage is dealt with, the better.

What global studies say

The World Health Organization (WHO), based on a meta-regression analysis of four of its vaccines granted emergency use authorisation (EUL) with the most data – Pfizer-BioNTech’s Comirnaty, Moderna’s Spikevax, Astrazeneca’s Covishield/Vaxzevria, and Johnson & Johnson’s Ad26.COV2.S – observed that there was a considerable decline in immunity, particularly among older people.

“Emerging data consistently show a decline in vaccine effectiveness against SARS-CoV2 infection and COVID-19 with time since vaccination, and more significant decline in older adults,” the United Nations health agency said in a statement on December 22.

Vaccine effectiveness against severe Covid-19 decreased by about 8% over a period of six months in all age groups, the analysis showed.

But the more alarming finding was that in adults above 50, protection against severe disease decreased by about 10% in the same period, and protection fell by 32% against symptomatic disease.

Importantly, this study was released by WHO before the emergence of Omicron and its sub-variants, which are much more infectious than other variants.

Another study by University of Edinburgh published in the Lancet journal gives us a much clearer understanding of immunity waning in Covishield. It looked real-world data from 44 million shot recipients – 42 million in Brazil and 2 million in Scotland.

Researchers started seeing a visible drop in immunity around three months after people were given their second shot.

Compared with two weeks after receiving a second dose, there was a fourfold rise in the chance of death and hospitalisation four months from the second dose, which became a fivefold increase in five months.

In the Indian context, the four-month period applies to the nearly 185 million people, or 53% of those above 45 years, while the five-month period applies to nearly 150 million people, or 43% of those above 45 years.

“Vaccines have been a key tool in fighting the pandemic, but waning in their effectiveness has been a concern for a while. By identifying when waning first starts to occur in the Oxford-AstraZeneca vaccine, it should be possible for governments to design booster programmes that can ensure maximum protection is maintained,” Aziz Sheikh, director of the University of Edinburgh’s Usher Institute and the study’s lead author, said in a statement.

“Our work highlights the importance of getting boosters, even if you’ve had two doses of the Oxford-AstraZeneca vaccine, as soon as you are able to,” added Vittal Katikireddi of the University of Glasgow, one of the co-authors of the study.

A third study in New England Journal of Medicine showed that vaccine effectiveness against symptomatic Covid-19 fell to 44.3% by the five-month point from the second dose for Covishield. Beyond the five-month point, effectiveness against hospitalisation fell to 80%, while against death to around 85%. Those over 40 and with underlying medical conditions saw a greater waning, according to this study.

Several factors must be taken into account when expanding booster ambit, such as coverage of all vulnerable groups, and the impact the Omicron wave would have had on the immunity levels, experts said.

“We have obviously prioritised for now the population that is most vulnerable to the virus – the elderly, health care and front line workers. There are several aspects of the strategy that the government is trying to look at right now – from having vaccine from multiple manufactures made available to covering children,” said Dr Vikas Maurya, director, pulmonology, Fortis Healthcare.

“I am certain they want to cover as this group (45 years and above) as quickly as possible. After these categories are well-placed, I definitely see boosters being expanded to the vulnerable people in the 45+ category as well in a month or two,” Dr Maurya said.

“There are also people predicting that a new wave may be on the horizon, even though it may not happen, in which case it would be best to cover as many vulnerable people with boosters as possible,” Dr Maurya said.

India’s third wave of infections caused by the Omicron variant may have bought India limited amount of grace time, he said.

“We also need to understand that the country just saw a massive Omicron wave in which many people exposed to the virus. In that sense, from what we’ve observed, that should buy them 3-4 months of immunity,” he added. “This would be a factor that would be in the minds of government experts when they’re planning out the booster strategy.”