AstraZeneca Scientist Says Delta Variant Makes Herd Immunity Impossible. Here's Why

The Delta variant has changed the equation for achieving herd immunity, the developer of the Oxford/AstraZeneca vaccine has said.

Speaking at a UK parliamentary meeting on Tuesday, Sir Andrew Pollard, a professor of pediatric infection and immunity at the University of Oxford, said that achieving herd immunity is "not a possibility" now that the Delta variant is circulating.

"We know very clearly with coronavirus that this current variant, the Delta variant, will still infect people who have been vaccinated, and that does mean that anyone who's still unvaccinated, at some point, will meet the virus," Pollard said.

He said it was unlikely that herd immunity will ever be reached, saying the next variant of the novel coronavirus will be "perhaps even better at transmitting in vaccinated populations".

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Vaccinated people can still get the Delta variant, albeit as a milder case

Some experts had hoped that herd immunity could be reached with COVID-19, as was the case with measles, which is also highly infectious.

Many countries have achieved herd immunity with measles by vaccinating 95 percent of the population against it, such as the US, where endemic transmission ended in 2000. That is because once a person is vaccinated against measles, they cannot transmit the virus.

With COVID-19, vaccines still fulfill their primary role: protecting against severe disease. According to the US Centers for Disease Control and Prevention, vaccinated people who catch the Delta variant are 25 times less likely to have a severe case or die. The overwhelming majority who do catch it will have mild or no symptoms.

But growing evidence suggests that, with the Delta variant, fully vaccinated people can still transmit the virus.

"We don't have anything which will stop that transmission to other people," Pollard said.

Israel is a good example of this: COVID-19 cases dropped in the country after it vaccinated about 80 percent of adults – prompting some to hope that it had reached herd immunity – but the Delta variant has since brought another surge of cases.

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