Five reasons why COVID herd immunity is probably impossible

Even with vaccination efforts in full force, the theoretical threshold for vanquishing COVID-19 looks to be out of reach.

As COVID-19 vaccination rates pick up around the world, people have reasonably begun to ask: how much longer will this pandemic last? It’s an issue surrounded with uncertainties. But the once-popular idea that enough people will eventually gain immunity to SARS-CoV-2 to block most transmission — a ‘herd-immunity threshold’ — is starting to look unlikely.

Nota original: Nature Christie Aschwanden https://www.nature.com/articles/d41586-021-00728-2

That threshold is generally achievable only with high vaccination rates, and many scientists had thought that once people started being immunized en masse, herd immunity would permit society to return to normal. Most estimates had placed the threshold at 60–70% of the population gaining immunity, either through vaccinations or past exposure to the virus. But as the pandemic enters its second year, the thinking has begun to shift. In February, independent data scientist Youyang Gu changed the name of his popular COVID-19 forecasting model from ‘Path to Herd Immunity’ to ‘Path to Normality’. He said that reaching a herd-immunity threshold was looking unlikely because of factors such as vaccine hesitancy, the emergence of new variants and the delayed arrival of vaccinations for children.

Gu is a data scientist, but his thinking aligns with that of many in the epidemiology community. “We’re moving away from the idea that we’ll hit the herd-immunity threshold and then the pandemic will go away for good,” says epidemiologist Lauren Ancel Meyers, executive director of the University of Texas at Austin COVID-19 Modeling Consortium. This shift reflects the complexities and challenges of the pandemic, and shouldn’t overshadow the fact that vaccination is helping. “The vaccine will mean that the virus will start to dissipate on its own,” Meyers says. But as new variants arise and immunity from infections potentially wanes, “we may find ourselves months or a year down the road still battling the threat, and having to deal with future surges”.

Long-term prospects for the pandemic probably include COVID-19 becoming an endemic disease, much like influenza. But in the near term, scientists are contemplating a new normal that does not include herd immunity. Here are some of the reasons behind this mindset, and what they mean for the next year of the pandemic.

It’s unclear whether vaccines prevent transmission

The key to herd immunity is that, even if a person becomes infected, there are too few susceptible hosts around to maintain transmission — those who have been vaccinated or have already had the infection cannot contract and spread the virus. The COVID-19 vaccines developed by Moderna and Pfizer–BioNTech, for example, are extremely effective at preventing symptomatic disease, but it is still unclear whether they protect people from becoming infected, or from spreading the virus to others. That poses a problem for herd immunity.The coronavirus is here to stay — here’s what that means

“Herd immunity is only relevant if we have a transmission-blocking vaccine. If we don’t, then the only way to get herd immunity in the population is to give everyone the vaccine,” says Shweta Bansal, a mathematical biologist at Georgetown University in Washington DC. Vaccine effectiveness for halting transmission needs to be “pretty darn high” for herd immunity to matter, she says, and at the moment, the data aren’t conclusive. “The Moderna and Pfizer data look quite encouraging,” she says, but exactly how well these and other vaccines stop people from transmitting the virus will have big implications.

A vaccine’s ability to block transmission doesn’t need to be 100% to make a difference. Even 70% effectiveness would be “amazing”, says Samuel Scarpino, a network scientist who studies infectious diseases at Northeastern University in Boston, Massachusetts. But there could still be a substantial amount of virus spread that would make it a lot harder to break transmission chains.

Vaccine roll-out is unevenz

The speed and distribution of vaccine roll-outs matters for various reasons, says Matt Ferrari, an epidemiologist at Pennsylvania State University’s Center for Infectious Disease Dynamics in University Park. A perfectly coordinated global campaign could have wiped out COVID-19, he says, at least theoretically. “It’s a technically feasible thing, but in reality it’s very unlikely that we will achieve that on a global scale,” he says. There are huge variations in the efficiency of vaccine roll-outs between countries (see ‘Disparities in distribution’), and even within them.

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