when people say that we won’t reach “herd immunity ” to covid-19, they are usually referring to an ideal of “full” population immunity: when so many people are immune that, most of the time, there is no community transmission.
with full herd immunity, most people will never be exposed to the virus. even those who are not vaccinated are protected, because an introduction is so unlikely to reach them: it will sputter out, because so many others are immune — as is the case now with diseases like polio and mumps .
for one thing, it appears that immunity to covid-19 acquired either by vaccination or infection wanes over time . in addition, sars-cov-2 will continue to evolve . over time, variants that can infect people with immunity (even if this only results in mild disease) will have a selective advantage, just as until now selection has mainly favoured variants with higher transmission potential.
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also, our population is a composition of different communities, workplaces and environments . in some of these, transmission risk might be high enough and/or immunity low enough to allow larger outbreaks to occur, even if overall in the population we have high vaccination and low transmission.
finally, sars-cov-2 can infect other animals . this means that other animal populations may act as a “reservoir,” allowing the virus to be reintroduced to the human population.
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some estimates suggest that we may need two thirds of the population to be protected either by successful vaccination or natural infection. if 90 per cent of the population is eligible for vaccination, and vaccines are 85 per cent effective against infection , we can obtain this two thirds with about 90 per cent of the eligible population being vaccinated or infected naturally.
the united kingdom has already exceeded these rates in some age groups . higher rates are even better, because there is still uncertainty about the level of transmissibility and vaccine efficacy against infection (although research shows they are very good against severe disease ). we don’t want to discover that we do not have enough immunity through vaccination and have another serious wave of infection.
that said, variants of sars-cov-2 will continue to emerge, and selection will favour variants that escape our immunity. vaccine developers will continue to broaden the spectrum of the vaccines that are available , and boosters will hopefully allow us to maintain long-term practical herd immunity.
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