Rhys Blakely, Science Correspondent CourtesyThe Times (UK)

Fully vaccinated people are far more likely to develop symptoms if they catch the Delta variant of the coronavirus compared with earlier strains, a large study has shown.

Research that looked at more than 700,000 fully vaccinated adults found that more than half of those who became infected with Delta developed classic” Covid symptoms, such as a cough, fever and loss of taste and smell.

With the Alpha variant, which fuelled the wave of infections over the winter, the equivalent figure had been less than 10 per cent.

The research also found that people who were fully vaccinated and those who were unvaccinated had the same levels of virus in their nose and throats if they became infected with Delta. With the Alpha variant, virus levels in those vaccinated had been much lower.

It was stressed that both the Pfizer-BioNTech and Oxford-AstraZeneca jabs still provided good levels of protection, especially against severe disease and hospital admission. However, the findings will raise concerns that the Delta strain, now dominant in the UK, has reduced the effectiveness of both vaccines at limiting the spread of the virus. Signs of waning protection will be weighed as government advisers discuss any booster-shot campaigns.

Professor Sarah Walker, of the University of Oxford, who led the study, said it was plausible that higher peak levels” of virus would mean that vaccinated people with Delta were more infectious. You still are less likely to get infected if you’ve had two doses — but if you do, you will have similar levels of the virus to somebody who hasn’t been vaccinated at all,” she said.

The study compared the protection offered by the Pfizer and AstraZeneca vaccines before and after mid-May — roughly the point at which Delta, which was first identified in India, became the main variant in the UK.

Before May 16, only about 7 per cent of vaccinated people who caught the virus showed symptoms. At that time, the Alpha strain, first identified in Kent, was dominant. After May 16, when virtually all cases were Delta, the number of vaccinated people showing symptoms jumped to 55 per cent.

The fact that we’re seeing more symptomatic infections with Delta and infections with higher virus levels does mean there is the potential for hospitalisations to increase, but I would really stress that we don’t know that yet,” Walker said.

More research would be needed, she added. We don’t yet know how much transmission can happen from people who get Covid after being vaccinated. For example, they may have high levels of virus for shorter periods of time.

But the fact that they can have high levels of virus suggests that people who aren’t yet vaccinated may not be as protected from the Delta variant as we hoped. This means it is essential for as many people as possible to get vaccinated, both in the UK and worldwide.”

The study also found that two doses of Pfizer is initially more effective at preventing new infections than two doses of AstraZeneca. However, the Pfizer protection declined faster. Fourteen days after the second jab, Pfizer was 85 per cent effective at preventing any infection, falling to 75 per cent on day 90. For AstraZeneca it was 68 per cent on day 14 and 61 per cent on day 90.

The study suggested that after four to five months, both vaccines are likely to perform about as well as each other, and that the interval between doses does not affect effectiveness in preventing new infections, which may potentially support the argument for the present eight-week gap to be reduced.

Dr Koen Pouwels, a senior researcher at Oxford’s Nuffield Department of Population Health, said: The fact that we did not see any effect of the interval between first and second doses, and the greater effectiveness of having had two doses rather than one, supports the decision to reduce [the gap] to eight weeks now that Delta is the main variant of concern in the UK.

However, while vaccinations reduce the chance of getting Covid-19, they do not eliminate it. More importantly, our data shows the potential for vaccinated individuals to still pass on Covid, and the importance of testing and self-isolation to reduce transmission risk.”

The study also showed that people who caught Covid and later received a vaccine had greater levels of protection. For example, 14 days after a second dose of AstraZeneca, infections had dropped by 88 per cent among those previously infected. For those who had not had the virus, it was 68 per cent.

With Pfizer, a prior infection led to 93 per cent protection; without the prior infection, it was 85 per cent.