Covid Delta variant offshoot ‘less likely’ to cause symptoms

An offshoot of the Covid Delta variant that is on the rise in England appears to be less likely to cause symptoms than the dominant form, researchers have revealed – although experts say the finding requires further scrutiny.

Scientists on the React-1 study analysed more than 100,000 swabs taken across England with the majority taken between 19 October and 5 November.

The results suggest that over that period, the infection rate for England was 1.57% – a figure that includes both symptomatic and asymptomatic infections – with prevalence highest among children aged 13-17, at over 5%. The team added that while rates were lower in older adults, levels had doubled in those aged 65 and over compared with the previous round of the study carried out in September.

While overall the infection rate in the latest study period was higher than in the September round of the study, the findings suggest there was a peak around 20-21 October, with the rate subsequently declining: results released earlier this month suggested the infection rate in the first half of the most recent study period was 1.72%.

The decline, the team add, reflects a fall in prevalence of infections among both children and adults below the age of 55 during overall study period.

The drop chimes with trends seen in data from other sources including the Office for National Statistics, although some have suggested it may be partly down to the impact of half-term.

Prof Christl Donnelly of Imperial College London and a co-author of the study added a similar dip was seen around the same time last year.

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Among other findings, the study revealed there has been an increase in infections among double vaccinated individuals in recent months, while an offshoot of the Delta variant, known as AY.4.2 and which is on the rise in England, appears to be less likely to cause symptoms than the dominant Delta variant, AY.4.

It has previously been suggested that AY.4.2 may be around 10-15% more transmissible than the original Delta variant.

But others said the new finding requires further scrutiny.

Dr Simon Clarke, associate professor in cellular microbiology at the University of Reading, said: “What the data doesn’t tell you is who those infections [of AY.4.2] are in. So if they’re in younger people, or if they’re in a community that has a higher than average vaccine uptake, then that might account for things,” he said.

Donnelly said a reduction in coughing may reduce spread, but suggested it is also possible a reduced tendency to cause symptoms may boost spread of the virus. “It is absolutely the case that if people are waiting for symptoms to do a test and to therefore identify that they are infected and therefore should cut back their contacts, being asymptomatic may facilitate transmission,” she said.

Clarke said: “It’s an interesting suggestion, but it’s not what’s been seen with the other [variants].”

Using linked health records, the team were also able to probe the impact of vaccination, revealing a single jab appeared to be 56.2% effective against infection, whether symptomatic or not, in children aged 12 to 17, two weeks after vaccination, and 67.5% effective against symptomatic infection.

Prof Paul Elliott, director of the React study, added the research also shed light on the impact of boosters.

“We found that within 14 days of having had a third dose, the risk of infection was reduced by two-thirds,” he said.

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