This has led to speculation about whether a population can achieve some sort of immunity to the virus with as little as 20% infected – a proportion well below the widely accepted herd immunity threshold (60-70%).
The Swedish public health authority announced in late April that the capital city, Stockholm, was “showing signs of herd immunity” – estimating that about half its population had been infected.
Hopes that the COVID-19 pandemic may end sooner than initially feared have been fuelled by speculation about “immunological dark matter”, a type of pre-existing immunity that can’t be detected with SARS-CoV-2 antibody tests.
Studies show that people infected with SARS-CoV-2 indeed have T-cells that are programmed to fight this virus.
This may lead to some level of protection against the virus – potentially explaining why some outbreaks seem to burn out well below the anticipated herd immunity threshold.
In the Italian COVID-19 epicenter in Bergamo, a town where one in four residents are pensioners, 60% of the population had antibodies by early June.
Early in the pandemic, there was much speculation about whether specific genetic receptors affected susceptibility to the SARS-CoV-2 virus.
This will likely drive contagion and cause new COVID-19 outbreaks in the weeks to come.
But this isn’t likely to be the case – young people, for example, are likely to have more acquaintances than the elderly.
Accounting for this reduces the herd immunity threshold to around 40%.
The lockdowns enforced far and wide, combined with the responsible actions of many citizens, have undoubtedly mitigated the spread of SARS-CoV-2 and saved lives.
That said, the fact that more than 20% of people have been infected in other places means that the T-cell hypothesis is unlikely to be the sole explanation either.
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