Why do the minority who haven’t had Covid account for most new infections?

The proportion of people who have never seen the red line appear on a rapid test is rapidly decreasing.

On Thursday, the White House announced that the US president, Joe Biden, had tested positive for Covid, becoming the most high-profile figure yet to join the club of people who are only now, in the third year of rife disease.

The figures from the UK health security agency are striking. Some people in England have never had Covid. The minority group still accounts for 45% of infections in the current wave. How can a small group of people contribute so much?

Multiple forces are working together with Covid.

In a textbook epidemic, the proportion of first-timers who get sick is expected to drop from 100% in the early days to a much smaller number as the bug becomes more widespread.

Babies and young children are the ones who are un exposed. We won't get to this point soon. More than 10 million people had never been exposed to Covid, according to the UKHA. Bill Hanage says it will take a while to get there.

As the Pandemic plays out, the decline in first time infections may not be as smooth. The downward trend is influenced by people's behavior, such as whether they protect or reduce social contacts.

Even though 45% of Covid infections in England are first-timers, this is the lowest it has been since the start of the epidemic. 45% of cases are reinfections, the highest figure has ever been, or at least very close to it.

First-time infections seem to be more common than the data shows. The worst infections with Covid tend to be the first ones. Many people on their second, third or fourth infections don't realize they have caught it again.

If infections after the first are less likely to besymptomatic, or less likely to be seen by the public as 'test worthy', then there will be many people who are being reinfecting but not noticing, so that first infections are more likely to be diagnosed and reported.

There are clues as to why more first time infections are occurring than in the first wave.

The first wave of Omicron sent case rates soaring for people in their 20s, 30s and 40s, but not as much for older people. As Boris Johnson said, the nation was bracing for a "tidal wave" of Omicron and mass testing was still in place, so people were prepared to avoid spreading Covid to older relatives.

The mass of younger people were protected by their recent infections when the next waves arrived in March and June.

A recent BA.1 infections is 80% protective against BA.4 or BA.5. The BA.2 and BA.4/BA.5 waves sent infections sky high in older age groups who are more likely to be first-timers.

After the first Omicron wave, plan B measures to curb the virus, such as working from home and compulsory mask-wearing, were dropped. It was easy to think that the epidemic was over for people who had not yet been affected.

Older people who contract the virus are more likely to spread it to younger people who are also first-timers. Hanage proposed a bridge club for the over 70s and a busy newsroom for the left.

Many people in the newsroom have been vaccined but also recently been bitten by the disease. There is only so much that can be done if a staffer brings an illness to work.

It's not the same at the bridge club. It's more likely that an infectious player will spread it to someone who hasn't been exposed before, and it's also more likely that they won't get sick. They are more likely to give it to other people.

This is bad news. Hanage says that the introduction of BA.5 is more likely to spread to groups that have not previously been affected. We can expect the serious consequences of infections in older people to be more frequent than if they were happening in younger people who have already been exposed.