Covid-19: How A lot Herd Immunity is Sufficient?

The study found that 1,064 of the 1,568 seafarers on board, or about 68 percent, had tested positive for the virus.

However, the airline returned to port during the outbreak and the crew was quarantined. Therefore, it was unclear whether the virus stopped infecting new sailors, even after 68 percent caught it.

Outbreaks on board ships are also a bad model for those on land, as infections move much faster near a ship than in a free-living civilian population, said Dr. Natalie E. Dean, Biostatistician at the University of Florida.

More importantly, the early estimates from Wuhan and Italy were later revised upwards, noted Dr. Lipsitch noted when Chinese scientists discovered they had under counted the number of victims of the first wave. It took about two months to make sure there were many asymptomatic people who had spread the virus as well.

It also later became clearer that “superspreader events,” where one person infects dozens or even hundreds of others, played a large role in the spread of Covid-19. Such events in “normal” populations, where no one wears masks and everyone participates in events such as parties, basketball tournaments or Broadway shows, can increase the number of reproductions to 4, 5 or even 6, according to experts. Consequently, these scenarios require higher herd immunity. For example, with an R0 of 5, more than four in five people, or 80 percent, must be immune to slow the virus down.

To further complicate matters, there is a growing consensus among scientists that the virus itself is becoming more transmissible. A variant of the “Italian strain” with the mutation known as D614G has spread much faster than the original Wuhan variant. A newly identified mutation, sometimes referred to as N501Y, has recently emerged in the UK, South Africa and elsewhere, which may make the virus even more contagious.

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