COVID-19 herd immunity may be an impractical strategy

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Achieving herd immunity to Covid-19 is an impractical public health strategy, say researchers, adding that, immunity is not perfect and achieving it through widespread exposure is very unlikely.

The study, published in the journal Proceedings of the National Academy of Sciences, investigated the suppression and mitigation approaches for controlling the spread of SARS-CoV-2, the virus that causes Covid-19.

"The herd immunity concept is tantalizing because it spells the end of the threat of Covid-19," said study lead author Toby Brett from University of Georgia in the US.

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"However, because this approach aims to avoid disease elimination, it would need a constant adjustment of lockdown measures to ensure enough people are being infected at a particular point in time," Brett added.

The research team sought to determine if and how countries could achieve herd immunity without overburdening the health care system.

They developed an age-stratified disease transmission model to simulate SARS-CoV-2 transmission in the UK, with spread controlled by the self-isolation of symptomatic individuals and various levels of social distancing.

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Their simulations found that in the absence of any control measures, the UK would experience as many as 4,10,000 deaths related to Covid-19, with 3,50,000 of those being from individuals aged 60-plus.

They found that using the suppression strategy, far fewer fatalities were predicted: 62,000 among individuals aged 60-plus and 43,000 among individuals under 60.

If self-isolation engagement is high (defined as at least 70 per cent reduction in transmission), suppression can be achieved in two months regardless of social distancing measures, and potentially sooner should school, work and social gathering places close.

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To instead achieve herd immunity given currently available hospital resources, the UK would need to adjust levels of social distancing in real time to ensure that the number of sick individuals is equal to, but not beyond, hospital capacity.

"If the virus spreads too quickly, hospitals will be overwhelmed, but if it spreads too slowly, the epidemic will be suppressed without achieving herd immunity," the team wrote.

They further noted that much is unknown about the nature, duration and effectiveness of Covid-19 immunity, and that their model assumes perfect long-lasting immunity.

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The team cautioned that if immunity is not perfect, and there is a significant chance of reinfection, achieving herd immunity through widespread exposure is very unlikely.

"We recognize there remains much for us to learn about Covid-19 transmission and immunity," said study authors.

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