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What is sufficient collective immunity?

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

But the carrier returned to port while the outbreak was still ongoing and the crew went into quarantine, so it was not clear whether the virus had finished infecting the new sailors, even after 68% l ‘had detected.

In addition, epidemics on board ships are bad models for those on shore, as infections move much faster in the close quarters of a ship than in a civilian population at large, said Dr Natalie E. Dean, biostatistician at the University of Florida.

More importantly, the first estimates from Wuhan and Italy were then revised upwards, Dr Lipsitch noted, once Chinese scientists realized they had underestimated the death toll from the first wave. It took about two months to be sure there were many asymptomatic people who had also spread the virus.

It also became clearer later that ‘mass media events’, in which one person infects dozens, if not hundreds of others, played a significant role in the spread of Covid-19. Events like this, in “normal” populations – where no one wears a mask and everyone attends events like parties, basketball tournaments or Broadway shows – can push the breeding number to 4. , 5 or even 6, according to experts. Therefore, these scenarios require higher collective immunity; for example, with an R0 of 5, more than four in five people, or 80%, need to be immunized to slow the virus.

To complicate matters further, there is a growing consensus among scientists that the virus itself is increasingly transmissible. An “Italian strain” variant with the mutation known as D614G spread much faster than the original Wuhan variant. A newly identified mutation, sometimes called N501Y, which can make the virus even more infectious, has recently emerged in Britain, South Africa and elsewhere.

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