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.

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.

Non-pharmaceutical interventions will continue to play a crucial part in keeping cases down, Del Valle says. The whole point is to break the transmission path, she says, and limiting social contact and continuing protective behaviours such as masking can help to reduce the spread of new variants while vaccines are rolling out.

But it’s going to be hard to stop people reverting to pre-pandemic behaviour. Texas and some other US state governments are already lifting mask mandates, even though substantial proportions of their populations remain unprotected. It’s frustrating to see people easing off these protective behaviours right now, Scarpino says, because continuing with measures that seem to be working, such as limiting indoor gatherings, could go a long way to helping end the pandemic. The herd-immunity threshold is “not a ‘we’re safe’ threshold, it’s a ‘we’re safer’ threshold”, Scarpino says. Even after the threshold has been passed, isolated outbreaks will still occur.


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