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To halt covid-19 in its tracks, we need vaccines that stop the virus spreading as well as preventing people becoming seriously ill. Until now it was unclear how effective the vaccines are at doing this, but with vaccine roll-outs well under way, we are starting to get some answers.

The good news is that mRNA vaccines like Pfizer/BioNTech appear to be around 90 per cent effective at blocking transmission. The bad news is that as there are no plans to vaccinate children under 16 anytime soon, and a relatively high proportion of adults globally who say they will refuse vaccination, this might not be enough to raise herd immunity above the threshold needed to halt transmission.

Vaccines can block transmission either by preventing people becoming infected or by stopping them passing the virus on even if they are infected. To grasp why blocking transmission is so important, imagine a vaccine that stops disease but not transmission.

In that case, the virus would just keep spreading and reach people who haven鈥檛 been vaccinated, or for whom the vaccine hasn鈥檛 been effective, leading to many more deaths. Vaccinating care home workers wouldn鈥檛 stop them infecting care home residents, for instance. And initiatives like vaccine passports wouldn鈥檛 stop people from picking up the virus overseas and bringing it home 鈥� including new variants.

鈥淭ransmission blocking matters enormously,鈥� says Marm Kilpatrick at the University of California, Santa Cruz.

It is very hard to measure transmission blocking directly. According to聽a 29 March , the closest we have got to doing this was a study in Scotland looking at infections in household members of 150,000 healthcare workers.

This study found that household members were 30 per cent less likely to become infected when the healthcare worker had received a single dose of either the Pfizer/BioNTech or Oxford/AstraZeneca vaccines. However, because household members may have been infected by people other than the healthcare worker, and the people in the study hadn鈥檛 received their second dose, this study probably significantly underestimates transmission blocking.

What can be measured more easily is infection blocking, by regularly testing people to see if they are infected even if they have no symptoms. According to a study in the US, the Pfizer/BioNTech and Moderna mRNA vaccines are .

It is possible that transmission blocking is even higher than infection blocking, for instance because people produce less virus if they are infected after vaccination, and are therefore less likely to infect others. Indeed, one study has shown that people who still get infected after one dose of the Pfizer/BioNTech vaccine聽.

However, Kilpatrick estimates that this fall in viral load would only reduce the risk of infecting someone else by 11 per cent. And the flip side of this study is that it shows that some vaccinated people still end up with high viral loads, he says.

He also points out that some other studies concluded the mRNA vaccines were only聽听辞谤听 effective聽at infection blocking, so transmission blocking might be a little lower than 90 per cent.

For other vaccines, we still don鈥檛 have good estimates. Nor do we know whether transmission blocking will fade over time, or whether it will be significantly lower against variants such as B.1.351 from South Africa and P.1, which is causing a major outbreak in Brazil, though this seems plausible.

What鈥檚 more, even 90 per cent transmission blocking might not be enough to get many countries past the herd immunity threshold.

Early in the pandemic, each person was infecting around three others, who then each went on to infect three others, and so on. This implies that in the absence of measures such as lockdowns, at least 2 in 3 people need to be immune to infection 鈥� or at least not transmit the virus even if infected 鈥� to halt the spread of the virus. In other words, nearly 70 per cent need to be immune to get R below 1.

With the more transmissible B.1.1.7 variant from the UK, each person may infect four or five others, suggesting the herd immunity threshold is at least 80 per cent. In many countries,聽 according to YouGov surveys, so reaching the herd immunity threshold via vaccination alone won鈥檛 be possible even if transmission blocking is 100 per cent and under-16s are vaccinated.

In the UK, for instance, 86 per cent of adults are willing to get vaccinated and 19 per cent of the population is under 16. Even if all the willing get vaccinated and transmission blocking is 90 per cent, there will still be under 70 per cent immunity.

Vaccinating all under 16s would get the level close to 80 per cent but not past it. If transmission blocking is lower than 90 per cent, we will be even further from the herd immunity threshold. This is partly聽why many models suggest聽there could yet be a third wave.

However, things aren鈥檛 quite as bad as this implies. Firstly, immunity acquired by natural infections also adds to herd immunity. According to the ECDC report, evidence from a number of studies shows that previous infection provides between 81 and 100 per cent protection against reinfection for at least five months. However, protection is lower in people aged over 65, and these studies were done before any of the new variants of concern became widespread.

Secondly, the closer a country gets to the herd immunity threshold, the harder it is for the virus to spread. Only a few additional measures, such as wearing face masks, may be enough to get R well below 1. In Israel, where B.1.1.7 now dominates and most people have now been vaccinated with the Pfizer/BioNTech vaccine, the R number rebounded to just over 1 soon after the first easing of lockdown, but it fell sharply as vaccination continued.

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