A plan to start offering Covid booster vaccinations in the UK from early September is extremely unlikely to happen, it is understood, given the concerns of the government’s vaccines watchdog about the clinical benefits and potential wider risks to vaccine confidence.

No formal date has been set for third vaccinations as clinical trials continue, but the Department for Health and Social Care and the NHS have talked about their starting early next month after results from a series of clinical trials due this month.

The Joint Committee on Vaccination and Immunisation (JCVI), which advises ministers, met on Thursday morning, however, and is expected to tell them that significantly more work is needed before booster jabs can be rolled out.

This would cover not only who might gain a net clinical benefit from another vaccination, but also complex and broader issues about whether the plan is even necessary at all and concerns about excessive focus on Covid jabs hampering public confidence in vaccination more generally.

“The jury is still very much out on what happens,” one JCVI source said. “One of the consistent things we have found is that when we undertake clinical trials, the results can be surprising.”

One study presented to the JCVI on Thursday from researchers at Oxford University indicated that if people are given a “mix and match” combination of vaccines for their first and second jabs, the side-effects can be worse, with potential implications for the idea of a using a different jab for a booster than someone’s first two.

Among other matters yet to be decided is who would benefit from a third injection. The is focus primarily on people who have suppressed immune systems, but even within this group there are many issues to be considered.

Another JCVI source said a particular worry was the potential impact of pushing third vaccinations on a public within which the take-up of other vaccinations, for example for children and teenagers, had “dropped off a cliff” since the start of the pandemic.

“The confidence in the Covid vaccines has been fantastic, but confidence is very fragile and we don’t want to harm it,” they said. “There is evidence that if someone has a reaction to a vaccine it can put them off having all vaccinations in the future.

“The most important message right now is to stress that anyone who has not had their first or second vaccination should do so.”

Other issues discussed at the meeting include the regulatory, legal and logistical hurdles of a third vaccination, many of which are different to those for the first two.

It is understood that some JCVI members are also concerned about the UK using large numbers of vaccine doses for third injections of limited clinical value when they could be sent overseas, but the consideration is not part of the committee’s remit.

The first source said that as far as the JCVI was concerned, any planning for a third vaccination had only been in case it was deemed necessary, and did not mean it would happen. “This is going to take more time and research. What we don’t want to do is make promises which then disappoint people,” they said.