Chennai: A recent increase in the share of the elderly in deaths from Covid-19 in three Indian locations for which data are available could be the starting point for a conversation around the possibility of waning immunity and the resulting need for vaccine booster shots, an IndiaSpend investigation shows.

Data on deaths from Covid-19 by age available for Kerala, Tamil Nadu and Mumbai show that the share of those aged 60 years and above in total Covid-19 deaths declined sharply from April 2021 onwards, with this age group becoming eligible for vaccinations from March 1 onwards. The initial government-recommended gap of four-six weeks between doses of both Covishield and Covaxin, the two Covid-19 vaccines approved for use in India at the time, made many seniors eligible for their second dose from early April onwards, coinciding with the sharp decline in the share of seniors in Covid deaths, which IndiaSpend reported in May 2021.

However, data now show that despite the curve of the pandemic being currently in decline in India, the share of the elderly in total Covid-19 deaths has risen slightly. While this increase is small, it has taken the share of the elderly in Covid-19 deaths back to levels last seen months ago, and is the first secular increase since the post-vaccination decline.

Both Kerala and Tamil Nadu have populations that are older than the Indian average, and advanced age is a strong predictor of death from Covid-19. However, these numbers are not simply a reflection of the demographics of these states. They show not just a high proportion of the elderly in deaths, but a particular trend: The share of the elderly in Covid-19 deaths first fell following those aged 60 years and above becoming eligible for vaccination, and is now rising again.

UK data show immunity may be waning in those vaccinated over six months ago

Experts suggest at least five potential explanations. One, there is the possibility that immunity acquired from vaccination among those aged 60 and above is waning first, given that more than six months have passed since many of them were fully vaccinated.

This is a likelihood that is now quite clearly established in the United Kingdom. Data from a Public Health England study on waning immunity in early September showed a distinct waning of immunity acquired from vaccinations, including from the AstraZeneca vaccine, branded Covishield in India and also India's most widely deployed vaccine, accounting for about 88% of all vaccine doses administered, as of November 12, 2021. In the second half of September, the UK rolled out a booster programme for everyone over the age of 50, and other vulnerable groups such as adults with comorbidities that put them in greater danger of severe illness.

"There is definitely the possibility of waning immunity, but for this to be established we would need data on the vaccination status of these recent Covid-19 deaths, and that is data we don't yet have," Gagandeep Kang, one of India's foremost virologists and a member of the National Technical Advisory Group on Immunisation in India, the apex advisory body on immunisation in the country, told IndiaSpend.

India must plug gaps in vaccination status in Covid-19 death data

Two, the rising share of the elderly in Covid-19 deaths could indicate that things are returning to "normal", Murad Banaji, mathematician at Middlesex University, London, who has been modelling the pandemic in India, particularly in Mumbai, told IndiaSpend. The relative advantage that early vaccinations granted the elderly compared to the immunologically naive (unvaccinated and without previous exposure to infection) younger population could be dissipating as the share of young people vaccinated grows, and the elderly are once again at the same level of risk relative to younger people as they were before vaccinations were rolled out, Banaji suggested.

The data on vaccination coverage do indicate that coverage among younger people has grown faster than among the elderly. First dose coverage among those aged 45-59 now exceeds that among those aged above 60 (only people aged above 45 who had certified comorbidities were vaccinated along with those above 60 years, beginning March 1; vaccination was opened to all aged 45 years and above on April 1). Fewer than half of all people aged above 60 years have had both doses of the vaccine, more than nine months after the first doses began to be rolled out.

Three, there could be unquantifiable changes in behaviour. Having remained relatively more protected through the first and second waves than younger people, the elderly could be returning to more social behaviours, which could be raising their relative risk, Vineeta Bal, noted immunologist and visiting faculty at the Department of Biology, Indian Institute of Science Education and Research, Pune, told IndiaSpend.

Since the spread of the virus depends not only on immunity but also on changing social dynamics around transmissions and the resulting exposure, it can be difficult to isolate one cause from the data, in the absence of a conclusive cohort study, Bal added.

Four, if the deaths among the elderly are being driven by the unvaccinated, the trend could point to the special vulnerability of the unvaccinated elderly. Unfortunately, the data on this too are limited. Data from Tamil Nadu show that 76% of intensive care unit admissions and 87% of Covid-19 deaths in August and September were among the unvaccinated.

Five, when there are relatively few deaths being recorded, it is hard to be certain of trends in mortality above the "noise", Banaji added. "To be clear, after India's deadly second wave, and with continuing vaccination, we would expect cases and deaths to decline across all age groups. But we don't know how long this will last. We need to keep an eye on what is happening in several European countries, which have been seeing sharply rising levels of infection despite good vaccination coverage. This has led to increasing deaths in some elderly populations, a possible reflection of waning protection."

So what does this mean for the conversation on booster doses in India?

"These data [on vaccination status of Covid-19 deaths] are essential to decide on the timing of booster doses for the elderly," said Kang. "From what we know about other severe acute respiratory syndrome coronaviruses, immunity is durable, and this is even though only antibodies have been measured, not even T-cell immunity or other pathways," said Bal.

While immunity will go down slowly in individuals and also in the population, there is no reason to assume immunity is waning and to rush to booster shots. Instead, the greater threat is from the unchecked spread of the virus in Africa with low vaccine availability. The potential of the virus to mutate grows there, and that is the greater threat since we now know that the virus can travel across international borders with ease," Bal added.

Karthik Madhavapeddi contributed to this story and Devangee Halder and Gangadhar Joshi contributed to the research.

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