The rate of healthcare workers testing positive for coronavirus dropped by 50% 12 days after they received the first Covid-19 vaccine dose, new data suggests.

Tim Spector, who is leading the Zoe Covid Symptom Study UK Infection Survey from King’s College London, said the findings were “fantastic news”, and indicated the real-life effect of a single dose.

The data focused on the healthcare workers who have been vaccinated, and are based on around 13,000 coronavirus swab test results per week.

There are 220,391 UK healthcare workers on the app.

It is fantastic news really that we were seeing this without having to wait three months to get a result

At a webinar setting out the findings, Christina Hu, head of product at the study, said 74% of contributors to the study who are healthcare workers have already logged a Covid vaccine.

She said the study indicates a significant reduction of around 50% in Covid positivity rate amongst those healthcare workers who did get their first vaccine dose.

“And this reduction of 50% remains consistent as the overall population’s Covid case numbers decline,” Ms Hu said.

Professor Spector said: “It is fantastic news really that we were seeing this without having to wait three months to get a result.”

He added that the numbers were going down because the study’s data suggests the rate of the virus has been going down in terms of new infections for the last four weeks.

Prof Spector said: “This reflects the fact that the risk, even for the unvaccinated, is slightly lower than it was at its peak at the beginning of January, and we’re seeing a very consistent effect in this group.

“So to get, I think it was 53% on average in that sort of two to four week window, after a single dose, is excellent news really.

“And I think this is the first sign that in real life, outside trials, what the effect is of a single dose.”

He added that there was the caveat that this only looked at people after 12 days.

Prof Spector said: “This brings up the other point that at no point after you have had a vaccine are you 100% protected.

“But particularly that first two-week period, there’s like zero protection.

“And many people might be much more relaxed and there’s some evidence that they aren’t as concerned about prevention of a vaccine, and might rush up to relatives and go on public transport more.

“So that’s a really important public health message for everybody, that actually the risks are really high just after you’ve had the vaccination but then it starts to improve.”

The professor of genetic epidemiology said the researchers were very interested to see if that 50% drops even further, by following the people in the study and them logging what’s going on between that period, up to 12 weeks.

He added: “Because this would seem to be vindicating the approach of the Government at the moment about the 12-week delay, if we can get at least 50% protection, very rapidly.”

Experts in the UK have said a second jab can be delayed for up to 12 weeks in a bid to get more people vaccinated sooner.

This approach was initially met with some hesitancy, with little data to support the move.

Studies have been ongoing to assess how effective vaccines are after a single dose and with a 12-week gap between doses.

On Tuesday, data released by the University of Oxford researchers indicated their jab developed with AstraZeneca offers protection of 76% up to three months and may reduce transmission by 67% – with efficacy rising to 82.4% after the second dose 12 weeks later.