Immunity against delta variant, high vaccination rate behind dip in cases

Naina Mishra

Tribune News Service

Chandigarh, July 7

From witnessing a Covid peak in the region two months ago, the UT and Punjab have shown a steep fall in daily cases and weekly positivity rate. Experts indicate that herd immunity against the most commonly circulating variant, Delta, as a reason behind this huge dip in the cases.

Chandigarh’s weekly positivity rate is 0.70 per cent, while Punjab’s is 0.4 per cent. The national positivity rate is 2.4 per cent.

Dr Amandeep Kaur Kang, Director, Health Services, UT, said, “It is for the first time that Chandigarh has shown such a low record of daily cases. Such a trend was not even seen after the first peak.”

Dr Kang believed that good vaccination coverage and herd immunity are two major reasons for the decline in cases in the city. “There are multiple factors for a steep fall in cases. To begin with, Chandigarh had imposed heavy restrictions during lockdown and enforcement was strict due to which cases started declining. Secondly, around 66 per cent population of the city has been vaccinated with at least first dose of the vaccine, which is also an indicator of the decline,” she said.

She added, “The sero survey report shared by the PGI shows that around 70 per cent children had natural immunity against Covid-19. Going by the report, the prevalence of infection among older adults will be even higher than children.

This implies that a majority of population got the infection in the second wave, during which the Delta strain was prominent and these people had gained natural immunity against the circulating variant (Delta) as well.”

The Delta strain was found in 95 per cent samples sent by the UT Health Department for genome sequencing on June 25.

Dr KK Talwar, Health Adviser to Government of Punjab, said, “In May, 90 per cent of samples sent for genome sequencing from Punjab had the Delta variant. The second surge happened due to this strain mainly. Many people got infected and we assume that many people have been asymptomatic as well. People have acquired immunity against this strain and herd immunity has kicked in. Delta Plus is now a new threat and scientific community is still finding out whether it can be dangerous or not. For now, Punjab has reported only two cases of the variant.”

Notable epidemiologist Dr Chandrakant Lahariya explained, “The disease spread is an outcome of interplay of immunity (natural or vaccine induced), Covid-appropriate behaviour of people and the characteristics of the virus (including variants). The current decline is largely attributable to natural immunity, as we see in sero survey findings and vaccine coverage. However, this cannot be taken as an assurance that this is enough to prevent all future spikes. We don’t really know how long immunity lasts after natural infection as well as vaccination, most likely for 6-9 months.”

Dr Lahariya added, “In months ahead, some of the currently protected population will become susceptible again, and then there can be increase in cases.”

"With this virus, we should not assume herd immunity, the way we see for a few other diseases. Higher the transmissibility of a strain, higher the proportion of population we need to achieve here immunity. With vaccine effectiveness against new variants with full dose ranging from 60-85%, the theoretical threshold of herd immunity appears to be more than 100% population. In other words, the concept doesn’t seem to be applicable for the ongoing pandemic,” he added.

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