THE number of days a person is infectious with the Delta variant is likely to be less than previous mutations, according to a virology expert.

Professor Wendy Barclay, Head of the Department of Infectious Disease at Imperial College London, said it was possible the risk would remain for four or five days, compared to eight with the Alpha variant that was first identified in the UK.

She said both the AstraZeneca and Pfizer vaccines had dramatically reduced severe disease but had been less effective in stopping infections of the Delta variant, which is now said to account for up to 97% of cases.

However, she said it was crucial that any potential boosters did not target this variant alone.

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The formula would be based on detailed analysis of all the mutations and "picking one that lies somewhere in the middle" she said.

She was speaking today to Professor Phil Spector of University College London, one of the founders of the ZOE tracking app, which is recording around 3000 new cases of Covid each day.

"What we can see is that the vaccines are still working really well and equally well for Delta and Alpha for keeping people protected from the severe end of the disease spectrum," said Prof Barclay.

"We are still in the 90% and above range.

"What's not working quite so well is protecting people from infection. 

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"We can see that the antibodies are not as good in the vaccinees at blocking the Delta. Luckily there are other parts of the immune system that work particularly well in preventing severe disease and hospitalisation. People can still get infected, particularly with single dose vaccines.

"I think what's going on it's different in a vaccinated person," she added. "They have their immune system primed and ready to go. That's actually going to make you feel a bit rotten."

"But I can well imagine that the respiratory infection is going to be better contained."

Prof Barclay said there is some concern that a bad winter flu season could "double the burden".

"If you have co-circulation we do have evidence from the first wave that a double infection could be particularly bad. 

"In the few documented cases where people were in hospital with both, they were pretty sick. That's a concern."

She said that given the current levels of cases, there was still a need for social responsibility "despite what the politicians say" to protect those who are vulnerable to infection.

"We are nearly there at the point of full release...there is a level of social responsibility. That means wearing masks, ventilation and keeping your distance"