Fewer than 2% of frail and elderly people admitted to hospital after a Covid-19 vaccine experienced coronavirus symptoms three weeks after a single dose of the jab, data shows.

The findings were described as “very good news” by Professor Calum Semple, a member of the Scientific Advisory Group for Emergencies (Sage), who worked on the academic paper.

It comes as Professor Adam Finn, from the University of Bristol and a member of the Joint Committee on Vaccination and Immunisation (JCVI), said the UK should expect another wave of coronavirus cases this summer.

Describing the findings of the vaccine study, which was published in March, Prof Semple told BBC Radio 4’s Today programme that there was a sharp drop off in the number of vaccinated people experiencing Covid symptoms 21 days after a single dose of a vaccine.

This is the point at which experts believe maximum immunity from a single dose becomes apparent.

In the study, most vaccinated people who displayed symptoms of Covid and went to hospital had become infected before their immune system had a chance to respond to the jab.

The paper looked at more than 74,000 hospital admissions between September and early March, of which just under 2,000 people had received the vaccine.

Prof Semple said experts then looked at how many days there were between receiving the vaccine and the onset of their Covid symptoms.

“Now if the vaccine didn’t work, that number of days would stay relatively constant over time, but instead, what you see is most people who were admitted had caught their infection within a week of vaccination – either side of the vaccination – but then there was a really sharp drop off in numbers,” he said.

I think the message here is that when you come away from clinical trials, we can still show that the vaccine is working in the real world

“Three weeks after being vaccinated, we could only count 32 people out of the 2,000 that had been vaccinated and that’s a tiny, tiny number – that’s less than 2%.

“And that’s just after the first vaccine, and that’s in your frail, elderly population.

“So this is really good real world data showing that this vaccine works and that one dose works really well.

“I think the message here is that when you come away from clinical trials, we can still show that the vaccine is working in the real world.”

Asked about the Government’s road map and the next set of easing of restrictions on May 17, Prof Semple said that “provided we get the vaccination rollout on time then I can see no reason why the road map isn’t adhered to.”

He added: “The problem is if further sections of society choose not to be vaccinated and then we get a second outbreak perhaps later in the summer, which is what has been predicted.

“The size of that will be very much dependent on how many people don’t get vaccinated.”

Meanwhile, Professor Finn said the Prime Minister was right that the UK will see a further wave of Covid-19 cases.

He told BBC Breakfast: “I’m afraid he is right yes. The models that we’ve seen on JCVI clearly point to a summer surge in cases as the lockdown is relaxed, because there are still many people in the adult population who’ve not been immunised and who will therefore start to transmit the infection between each other.”

He said there was “quite a wide range of uncertainty” over how big the wave will be “because it depends on how quickly the vaccine rollout continues forward, the supplies of vaccine and so on, and how many people come forward to receive vaccination, and also it depends on how people behave as the lockdown is gradually relaxed”.

“If people move too far forward with that too fast, we’ll see things start to come up earlier,” he added.

“The sense that the problem is all over, I’m afraid… is a flawed one, we’re still in a vulnerable situation, and there are still significant numbers of people who potentially could be harmed by this infection if this happens.”

Asked if the further easing planned for May 17 may need to be adjusted, Prof Finn said: “This is a balancing act, isn’t it?

“People want to have some kind of certainty and businesses want to know how to plan, but on the other hand I think it’s always been presented as as a provisional timetable, based on what actually happens.

“I think if we do start to see significant rises in cases in some parts of the country, they may need to adjust back those dates in order to avoid the situation coming into effect. It’s a bit hard to be definite about this because, by definition, it’s uncertain.”

The leading expert said he was “somewhere in between” on what he thought would happen with vaccines and their effectiveness against variants.

“I don’t think that we’re going to see a complete collapse and ‘back to square one’ situation,” he said.

“I think that the immunity that we’ve got already from infection and vaccines will continue to be useful, but it will get eroded and there will come a point where we need to reformulate vaccines to keep up with changes in the virus.”

He said real-world data from Israel and the UK “where there are a lot of people who’ve been immunised” will show whether jabs work against variants.

Experts will be “looking very hard to see if there are any cases occurring among people who’ve been immunised and whether these particular variants are more likely to show up in that context”, he added.