On Tuesday, Andrew Pollard—the director of the Oxford Vaccine Group, which partnered with AstraZeneca to develop a widely used coronavirus vaccine—said he no longer thinks it's possible for populations to develop herd immunity against Covid-19.

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'Herd immunity is not a possibility'

Public health officials have long cited herd immunity as the ultimate endpoint of the pandemic. In a state of herd immunity, even "those who are not vaccinated … are protected by the overall level of immunity present in a population," CNBC reports.

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Experts' estimates have varied, however, as to the rate of population-wide immunity required to achieve that goal—in part because the coronavirus has evolved to become more infectious over time.

For instance, the World Health Organization last summer suggested that herd immunity likely would be reached when 60% to 70% of the population had been infected or vaccinated. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases and chief medical advisor to the White House, suggested the threshold in the United States would be around 75% to 85%.

However, Fauci later shied away from herd immunity as a goal due to the significant proportion of unvaccinated Americans.

This week, Pollard went a step further, saying that—in large part because of the delta variant—"herd immunity is not a possibility because it still infects vaccinated individuals."

CDC has cited numerous instances of breakthrough infections with the delta variant in vaccinated individuals. The agency notes, however, that the vast majority of breakthrough infections are mild and emphasizes that vaccination is still the best way to prevent Covid-19.

Pollard said Covid-19 vaccines might slow the spread of the virus because vaccinated people who get infected appear to shed less virus. But he warned that even more contagious variants are likely to emerge. "I suspect that what the virus will throw up next is a variant which is perhaps even better at transmitting among vaccinated populations, and so that's even more of a reason not to be making a vaccine program around herd immunity," Pollard said.

He argued leaders should stop focusing "on what might stop new variants because I don't think we have any facility to control that," and instead "focus on thinking about how do we prevent people from dying or going to [the] hospital."

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Specifically, Pollard argued that the United Kingdom should "play a more active role in the global imperative, which is to stop people from dying. … That means making sure [vaccine] doses are going to the right people."

Separately, Danny Altmann, a professor of immunology at Imperial College London, said the notion of herd immunity has been "contorted and over-simplified" as an endpoint for the Covid-19 pandemic. However, he noted that the "more people on the globe effectively vaccinated, the fewer viral copies we'll have on the planet, thus the less spread and fewer lungs in which for virus to mutate and spread the next wave of variants."

Promising new data from the UK

Yet even though herd immunity might not be possible against the delta variant, some countries have still had success in fighting back the variant.

For instance, the United Kingdom recently reported a reversal of its delta-driven Covid-19 surge—a trend that has spurred "optimism among doctors and scientists" that delta "can be held at bay with high levels of vaccination and public caution," the Wall Street Journal reports.

Although caseloads have increased since Prime Minister Boris Johnson dropped most public-health restrictions in July, the Journal reports, hospital admissions have been falling, and deaths are now a fraction of levels seen in earlier phases of the pandemic.

According to scientists, that decline likely stems from widespread immunity among the U.K. population. The U.K. statistics office estimates that 90 to 94% of British adults have some degree of Covid-19 immunity, either from full or partial vaccination, or prior infection. Overall, 59% of the population, and nearly 75% of adults, are fully vaccinated, compared with roughly 50% and 61% in the United States, respectively.

Another factor, according to scientists, could be better adherence to precautionary measures. Roughly 70% of Britons wear masks in public places, the Journal reports, whereas less than half of Americans reportedly do so.

Paul Hunter, an epidemiologist and professor of medicine at the University of East Anglia, said he thinks the United Kingdom is approaching "a state of endemic equilibrium," where vaccines mean the virus isn't likely to fuel significant new waves of sickness, but it could still cause sporadic outbreaks.

Still, Pollard in his comments suggested that such a situation falls short of the promise of true herd immunity, and encouraged further vaccination. "[A]nyone who's still unvaccinated, at some point, will meet the virus," he said. "That might not be this month or next month, it might be next year, but at some point they will meet the virus, and we don't have anything that will stop that transmission." (Douglas/Shah, Wall Street Journal, 8/11; Ellyatt, CNBC, 8/12; Schnell, The Hill, 8/11)