The UK government has commissioned a study to investigate the effects of Covid-19 vaccination on transmission of the virus, which will play a big role in Boris Johnson’s decision on when to ease England’s lockdown.

Coronavirus vaccines have been found to have a high degree of efficacy in providing immunity from the disease, but their impact on transmission of the disease is less clear. Downing Street officials said cutting transmission was a “critical factor” in easing the current restrictions.

The study, which is being overseen by Public Health England, is focused on frontline healthcare workers who were given jabs early in the vaccination programme. Jonathan Van-Tam, deputy chief medical officer, is closely involved with the research, which is expected to conclude in late February.

Prof Van-Tam confirmed at a Downing Street press conference on Wednesday that the research was under way and it would be a question of “to what extent”, not if, vaccines cut transmission rates.

Patrick Vallance, England’s chief scientific officer, said: “You don't have vaccines of this efficacy without there being some effect on transmission,” adding that this research would “determine to what extent these vaccines can be used across wider society to reduce transmission overall".

One Whitehall official said the research was vital to the lockdown easing. “We can’t decide how quickly to ease lockdown until we know by how much the vaccines cut transmission. That’s going to be critical to where we head next.”

Another senior insider confirmed that the PHE study on transmission would feed into the government’s review of the lockdown measures, set to take place on February 15. The results will be published the following week and no relaxation is expected before early March.

There is scant data so far on how vaccinations affect infection rates. Preliminary findings from Israel, which has led the world in vaccinating its population, suggest that the BioNTech/Pfizer vaccine has reduced infection rates. Data from Israel’s vaccination programme will be part of the PHE review.

PHE’s Siren study has since June been regularly testing tens of thousands of healthcare workers across the UK for new Covid-19 infections, as well as the presence of antibodies, which suggest people have been infected before.

Susan Hopkins, PHE’s strategic response director who will lead the new research, said: “The Siren study is one of a range of ways that we are assessing vaccine effectiveness. We will be sharing our findings with the JCVI [Joint Committee on Vaccination and Immunisation] to help inform decision-making and further information will be available in due course.”

The study has found that people infected with the virus in the past are likely to be protected against reinfection for at least five months, but earlier this month PHE said experts had cautioned those with immunity “may still be able carry the virus in their nose and throat and therefore have a risk of transmitting to others”.

PHE said that in order to directly monitor the effect on transmission, healthcare workers and care-home staff identified through the Siren programme, as well as members of the public, would be recruited.