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A paper by Oxford University researchers suggests that the UK population may already have developed sufficient levels of herd immunity required to prevent a second wave of the coronavirus pandemic in the country. The paper, titled ‘The impact of host resistance on cumulative mortality and the threshold of herd immunity for SARS-CoV-2’, is pending peer review and is currently online on a preprint server.

Epidemiologist Sunetra Gupta and three other Oxford University colleagues base their conclusion on the suggestion that the herd immunity threshold (HIT) required to prevent a resurgence is lower than it is taken to be. “It is widely believed that the herd immunity threshold (HIT) required to prevent a resurgence of SARS-CoV-2 (COVID-19) is in excess of 50 per cent for any epidemiological setting. Here, we demonstrate that HIT may be greatly reduced if a fraction of the population is unable to transmit the virus due to innate resistance or cross-protection from exposure to seasonal coronaviruses,” notes the paper.

Herd immunity can be calculated based on a homogeneous or a heterogeneous model. The homogeneous model assumes that everybody has the same level of immunity. The heterogeneous model, which has been the basis of a number of studies on SARS-Cov-2, assumes differing levels of immunity within the population.
The new paper works with a model that has found that when resistant people mix with non-resistant people, the threshold drops sharply. “The drop in HIT is proportional to the fraction of the population resistant only when that fraction is effectively segregated from the general population; however, when mixing is random, the drop in HIT is more precipitous. Significant reductions in expected mortality can also be observed in settings where a fraction of the population is resistant to infection. These results help to explain the large degree of regional variation observed in seroprevalence and cumulative deaths and suggest that sufficient herd-immunity may already be in place to substantially mitigate a potential second wave,” it says.
The study comes days after King’s College London scientists, in another preprint paper, suggested antibody-mediated immunity to SARS-CoV-2 infection may be short-lived. They analysed the immune response of 90 recovered individuals, and found antibodies against SARS-CoV-2 decreased during an 18-65 day follow-up period.

Excerpted from https://indianexpress.com/article/explained/has-uk-reached-herd-immunity-threshold-to-prevent-2nd-wave-6511173/