The Delta variant is the new pandemic scare story. Its spread into the United States has not led to a surge in deaths or packed hospital wards, but the news of its existence rained on the Fourth of July parades.

Public-health specialists have responded with a baffling spray of contradictory recommendations, conflicting information and seemingly inconsistent scientific facts. The result, as in earlier phases of the Covid-19 scourge, has been needless confusion and rage. Should vaccinated people wear masks? It depends whom you ask. Can economic life return to normal? Expert opinions vary.

Some say it depends on local conditions. Anthony Fauci, the White House medical adviser, says everyone should mask up in Alabama, which has a low vaccination rate and climbing cases.

Rochelle Walensky, director of the US Centres for Disease Control and Prevention, recommended indoor masking for any community with a vaccination rate under one third. But Los Angeles County wants vaccinated people to wear masks despite a relatively high vaccination rate.

Delta, first detected in India, is even more transmissible than the super-transmissible variant now called alpha — previously B.1.1.7 and originating in Britain — which prompted calls for double masking just last spring.

The Delta variant arose as a threat around the same time that public-health authorities in the US were encouraging individuals to make their own decisions about coronavirus safety instead of urging uniform standards of behaviour. In May, the CDC relaxed its masking recommendations and Walensky declared that fully vaccinated people “can start doing the things” that they had stopped doing because of the pandemic.

That shift was prompted by availability of highly effective vaccines, which appear to work against all forms of Covid-19, including the Delta variant. But a shift to a focus on personal responsibility is also a political and ethical change. So it is unsurprising, if also unsatisfying, that recommendations and rules vary: the World Health Organisation joins Los Angeles County in saying that vaccinated people should wear a mask in most indoor settings, but the CDC maintains that mask decisions should be up to regions, businesses and individuals.

Walensky explained that vaccines are much more available in the US than in most parts of the world and are more widely used in some US states than others. So the CDC and WHO recommendations both can be said to have scientific support. But science also shows that vaccinated people are highly unlikely to get seriously ill even if infected by the Delta variant, and are extremely unlikely to infect others in the grocery aisles.

That is why “the science” isn’t, by itself, a prescription for sound public-health policy. Virology and epidemiology can give people an estimate of how risky something is, but cannot tell people how much risk is too much, or which mitigation measures impose too much burden.

Communication separating the scientific from the political has not been clear, and unease about unmasked faces showed in the comments section of a recent Boston Globe article telling people in Massachusetts they should respect but not fear the Delta variant.

The story quotes Shira Doron, hospital epidemiologist at Tufts Medical Centre saying, “There is no scientific reason why a vaccinated person should wear a mask anywhere, except for their own comfort.” Outraged commenters reiterated a belief that masks are the best way to protect others.

Much of the public’s continued fear, and the public-health guidance that reflects it, she said, might trace back to misguided warnings early in the vaccination campaign that vaccines did not protect others.

Now there is data showing that the risk of getting infected at all drops by 60 per cent to 80 per cent after becoming fully vaccinated, said Muge Cevik, an infectious disease doctor at the University of St Andrews in Scotland. And those who do get a breakthrough case have lower odds of transmitting it to others. That doubtless explains why hospitals did not become overwhelmed when the Delta variant became dominant there, and why deaths aren’t surging.

Who still got infected? Cevik said it was primarily younger people who had not been vaccinated, as well as a few vaccinated people with what she considers the highest risk occupations — taxi and bus drivers and people in the hospitality industry. The risk comes from long-duration indoor exposure to a large number of people every day. And remember: hardly any of the infected vaccinated people became seriously ill.

Proponents of universal masking considered it a worthwhile strategy in 2020, when cases were rising and vaccines were not widely available, but how much masks helped is still not well quantified.

Two of the doctors who pushed hard for mask-wearing back then told me it was no longer needed in the US by May 2021. One of them, Monica Gandhi, of the University of California at San Francisco, said that vaccines are so effective that she would not recommend going back to universal masking unless test positivity rate climbed above 8 per cent — much higher than the present rate in Los Angeles.

Cevik said the biggest trouble from Delta is going to show up in countries with low vaccination rates. The main effect in the US will be on the unvaccinated because its increased transmissibility will mean they are less likely to be indirectly protected by herd immunity.

But in the states where more than half the population is vaccinated, and with rates among those with vulnerabilities of age or disease much higher, experts do not expect a new wave of deaths or overcrowded intensive-care units.

It is still theoretically possible for some vaccinated people to develop long-term complications from a mild case, or that some serious cases will show up among people taking immune-suppressive drugs. So the risk has not been eliminated. But Delta isn’t setting us back to 2020, either.

The confusion comes from this shift to personal responsibility not being made clear, any more than the shift away from it was made clearly enough back in the spring of 2020.

There was a political element to the shift both times — and that’s fine as long as it is informed by good science and it is what people want.

• Faye Flam is a Bloomberg Opinion columnist and host of the podcast Follow the Science