Numbers Matter | The figures behind the booster expansion debate

Protection against severe Covid starts fading fast after 4-6 months. This is why, the sooner the issue of wider booster coverage is dealt with (and boosters are made available), the better for India, and the world.

As of March 20, 2022, nearly nine out of every 10 Indians (87%) above the age of 45 have now received two shots of the Covid-19 vaccines. In the 18-45 years cohort, the proportion of people who have already received both shots of the vaccine is around 77%, according to government data seen against the Census of India’s population projection. In terms of single-dose coverage, the proportion covered for the 45+ age group and the 18-45 age group is 95% and 93% respectively.

Several global studies, both in a laboratory setting as well as those that looked at real-world data, have shown that vaccine effectiveness against Covid-19 starts decreasing after a certain time period. (Frank Hoermann/SVEN SIMON/picture alliance)

This means that, as things stand, very few adults in the country are yet to receive their Covid-19 vaccine shot — a remarkable achievement for a country the size of India. But looking at when these people got their second shots presents a clearer of the challenges that still remain.

First, let’s look at the 45+ age group. A little over a third (36%) of all Indians in this age category were administered their second shot at least six months ago (before September 20, 2021). In a month from now, this group will grow to cover 43% of all people above the age of 45. In two months, half a year would have elapsed since the day more than half of all Indians (53%) above the age of 45 would have received their booster shot.

Understanding the timing of vaccine coverage for this age group, in particular, is crucial because studies (and several experts) have stressed on the role played by immunity waning around the six-month mark — highlighting the need for India to make available booster shots to this population cohort, and progressively to everyone over 18 years old. Currently, only those over 60 are eligible for boosters in India among the general population (boosters are also available for health care workers).

Several global studies, both in a laboratory setting as well as those that looked at real-world data, have shown that vaccine effectiveness against Covid-19 starts decreasing after a certain time period, widely believed by experts to be around four to six months since the administration of the second dose. These studies have also shown that people over the age of 45, especially those who have comorbid conditions, are particularly prone to severe disease after the six-month mark.

These studies have been consistent in their messaging — protection against severe Covid starts fading fast after 4-6 months.

On December 22, 2021, at a time when the world was at the cusp of Omicron waves, the World Health Organization released a statement on booster doses for Covid-19 vaccination. The United Nations health agency analysed data from four of the world’s most heavily used vaccines — Pfizer-BioNTech’s Comirnaty, Moderna’s Spikevax, Astrazeneca’s Covishield/Vaxzevria, and Johnson & Johnson’s Ad26.COV2.S — and observed that there was a considerable decline in immunity with time.

“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,” it said.

It said that in adults above 50, protection against severe Covid-19 decreased by about 10% at the six-month mark, and protection fell by 32% against symptomatic disease.

In another study, which analysed the immunity waning characteristics of Covishield (which is India’s most-used vaccine, responsible for nearly nine in every 10 shots administered in the country), researchers at the University of Edinburgh started seeing a visible drop in immunity around three months after people were given their second shot. This study relied on real-world data from 44 million people administered Covishield shots in Brazil and Scotland.

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 jump in five months. To repeat the numbers stated above, for the Indian context, the four-month period currently applies to 43% of all Indians above the age of 45, while the five-month period applies to 53%.

A third study published in the New England Journal of Medicine, which also analysed Covishield’s immunity waning along with that of the Moderna mRNA shot, found that effectiveness against symptomatic diseased fell to 44.3% by the five-month point from dose 2. Beyond this, 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.

“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,” wrote Aziz Sheikh, director of the University of Edinburgh’s Usher Institute and the lead author of the second study quoted above.

“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.

Experts have repeatedly stressed that such studies add to mounting arguments in favour of theexpansion of the booster drive. And while the 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.

The onset of Omicron wave may have not led to spike in deaths, but a major role in containing fatalities in this wave compared to previous waves was the protection provided by vaccines. And if this protection starts to fade, and no boosters are provided, then people may not be so lucky in future waves.

If the pandemic has taught us anything, it is the value of data. To help understand the battle against the pandemic, Jamie Mullick, HT’s Covid data whiz, writes Numbers Matter

The views expressed are personal