Lockdown has slashed the rate of Covid-19 infection in France. That’s the good news. The bad news is that herd immunity is therefore a distant dream and a second wave of infections is pretty likely.

The number of people infected by someone who has contracted the virus has fallen from 3.3 to 0.5 since lockdown was declared, according to a study by researchers from the very authoritative Institut Pasteur, the national health authority and the medical research institute, Inserm.

The number of people admitted to intensive care fell from 700 a day when lockdown began to no more than 200 by 14 April. That means there should be between 1,370 and 1,900 intensive-care patients on 11 May, when the confinement is to be relaxed, compared to 5,433 today.

But confinement has also meant fewer overall cases, so fewer people immunised.

The researchers estimate that 3.7 million people living in France will have come in contact with the virus by 11 May. That sounds like a lot but it’s only 5.7% of the population. Scientists believe that 70% of the population need to have been infected to attain herd immunity.

Even in the worst-hit parts of the country, Ile de France and the Grand Est, the proportion infected are only 12.3% and 11.8% respectively.

On seeing the headline, I hoped the researchers had based their estimates on the official figures of the infection rate, which are complete nonsense due to the failure to carry out widespread testing.

Unfortunately, they’re really serious scientists, so they were much more methodical than that.

They based their estimates on the number of deaths in France so far, compared to the ratio of deaths to infection aboard the cruise ship Diamond-Princess, which was placed in quarantine off Japan. Given that all the passengers were tested, the figures should be reliable after being adjusted for variables, notable age.

The study finds that you have a 0.53% chance of dying if you catch Covid-19, which confirms findings of 0.5-0.7% in China. As we already know, there are great variations according to age – there’s only a 0.001% chance of dying if you’re under 18 but an 8.3% chance if your over 80 – and some according to gender, men being more likely to die than women.

The researchers also estimate that only 2.6% of people who catch the virus end up in hospital – lower than an estimate of 4.5% based on Chinese figures – and 18.2% of them have to be placed in intensive care.

So a second wave is very likely, either soon after lockdown ends if people are not as careful as they are now, or in the autumn, if the change in the weather and the arrival of other flu strains gives a new lease of life to Covid-19.

“Population immunity appears insufficient to avoid a second wave if all control measures are released at the end of the lockdown,” is the researchers understated comment.

On the other hand, checking the spread of the virus has given scientists time to research possible cures and look for a vaccine.

But, according to Prime Minister Edouard Philippe, a vaccine is unlikely to be developed during the course of this year. So we have many anxious months ahead of us.

France’s Covid-19 death toll is now officially 20,796, with 531 people dying in the past 24 hours. 30,106 people are in hospital, down 478, and 5,433 in intensive care, down 250. 39,181 people have been discharged from hospital, 1,772 over the last 24 hours.