Low transmission rates in the UK mean there is “little chance” of trials in the country proving the effectiveness of a coronavirus vaccine, a scientist leading a UK trial has said.

Professor Sarah Gilbert, who is leading the University of Oxford vaccine trial, said that when Covid-19 transmission was high, lockdown was imposed to bring the rate down.

Rates have since dropped, but a sufficient number of volunteers have to be exposed to the virus to see whether a vaccine protects them or not.

However, if their chances of being in contact with an infected person are low, it will take a long time to demonstrate the efficacy of a vaccine candidate.

We now have essentially a very large safety immunogenicity study, running in the UK with little chance, frankly, of determining efficacy

Prof Gilbert told the House of Lords Science and Technology committee: “Of course, what happened was that because it [transmission] actually increased much more rapidly than anybody thought was going to happen, we had the lockdown, which fortunately reduced transmission.

“Not so fortunate for those of us trying to develop vaccines in the UK, because we now have essentially a very large safety immunogenicity study, running in the UK with little chance, frankly, of determining efficacy.”

She added that the researchers’ approach was to work with multiple different countries in different settings to give them the best chance of seeing efficacy in at least one of those countries.

The committee heard the Oxford team is already running a trial in Brazil because the numbers of transmissions are much higher than in the UK.

“As soon as we get a signal of efficacy and can compare that to the level of immunity that we’re generating, that gives all vaccine developers really helpful information to let them know whether their vaccines are likely to work as well, and whether it will be one dose or two doses, and in older people and in younger people,” she said.

“So the first efficacy signal is going to be really important and as yet we don’t know which country will be generating that.”

Prof Gilbert and Professor Robin Shattock, who is leading the Imperial College London trial, said they were optimistic about their vaccine approaches but cautioned that optimism should be balanced against known risk of success.

Prof Shattock said vaccine success rates “tend to be at about 10% once you start clinical testing”, but added: “I think in the UK we are very lucky that we have two [vaccine] candidates that are already in clinical evaluation.

“We think that probably both of those will work individually

“We also have the opportunity to have a look at them in combination which I probably think nobody else currently globally is thinking about.”

Prof Gilbert agreed with Prof Shattock saying that, if necessary, “we can combine the vaccine to get something that works even better”.

She added: “The aim is to protect the population and that doesn’t mean the vaccination has to be 100% effective – even with a 50% efficacy, we could actually go a long way to protect the population.”