With nine in 10 adults in England now carrying antibodies against Covid, and infections seemingly in retreat, it would be tempting to think the country’s epidemic is all but over.
Yet there is growing evidence that the delta variant is far better than previous strains at reinfecting people who have previously had the virus or been double jabbed.
On Friday, Public Health England (PHE) upgraded its risk assessment for reinfection after a natural infection from amber to red, warning that the delta strain more than doubles the risk of getting Covid for a second time when compared with the alpha variant.
So are we about to see all our hard work wiped out by a wave of reinfections?
Weekly rate of possible reinfection
It might sound alarming – but it is important to realise that the chance of catching Covid for a second time is very small, so even a doubling brings little cause for concern.
In England so far, there have been just 23,105 cases of possible reinfection out of 4.3 million infections – 0.5 per cent of the total, or one in 200.
Comparatively, some 1.2 per cent of the 83,197 delta cases analysed were in people who previously had the virus – one in 83. It means more than 98 per cent of delta infections are new, so the strain is clearly not rampaging through an immune and vaccinated population.
The percentage of reinfections as a proportion of infections has also been falling since May, when it peaked at 4.4 per cent, and is now currently around 1.6 per cent.
Reinfections as a proportion of total first infections
Many of these cases may not even be true reinfections but rather the re-emergence of the previous illness that was not completely cleared. It is only possible to tell a true reinfection by genetically sequencing the strain, which is not always done.
Instead, reinfection is defined as an illness caught 90 days after being diagnosed with a first infection, or following four negative tests.
There are also reasons to be hopeful that, even if people do get reinfected, their illness will not be severe.
An analysis by the Office for the National Statistics (ONS) in June found that the majority of reinfected people get a mild case. Research showed that although one in 142 caught the virus again, just one in 1,000 got a bad case with a high viral load.
In fact, many did not experience symptoms at all the second time around, with just 21 per cent suffering problems such as fever, headache and fatigue compared to 57 per cent during their first bout of illness.
However, it does appear that women are more affected by reinfection than men in all age groups, with rates approximately double in 30 to 60-year-olds, which could represent differences in immune systems, employment or social mixing.
Rate of reinfection in England by age and gender
It is unclear why delta may be causing more reinfections, but it may be that immunity from infections early in the pandemic is starting to wane, reducing the body’s defences.
A recent Lancet paper showed that vaccine antibodies do start to wane over time, reducing by up to five-fold 70 days after a second dose of the AstraZeneca jab and two-fold for Pfizer.
It could also be that the variant itself is better at escaping from immunity.
So far, no data has been released about whether vaccination helps protect against reinfection for people who had a previous bout. But there are signs that it will, as people who have previously been infected appear to develop a better immune response after a first vaccination than those who never had the virus.
In fact, just one dose of vaccine generated antibodies equal to – or higher than – those produced by two doses of vaccine in people without previous infections, leading experts to suggest they may not need a second dose.
Real-world data shows that vaccines are generally holding up very well against the delta variant, with AstraZeneca preventing 67 per cent of infections and Pfizer 88 per cent.
So about three people in 10 who get vaccinated with AstraZeneca and are exposed to Covid will have a breakthrough infection, with about one in 10 who have the Pfizer jab. Out of nearly 230,000 Delta infections so far, just 12 per cent have been in double-jabbed people.
Among the over-50s who have received both vaccinations there have been 220 deaths out of 13,427 cases – 1.64 per cent. In contrast, there have been 131 deaths in unvaccinated over-50s among 2,337 cases – 5.6 per cent.
All this suggests that prior immunity and vaccination will be enough to provide a good defence against delta variant and that even if people are reinfected they are unlikely to be severely ill.
It is also likely that, by the autumn, we will have tweaked vaccines able to give even more protection against variants, providing greater reassurance that we do, finally, have a path out of the pandemic.