Plans for a Covid booster jab programme will be published "in due course", a senior government source has said."Extensive" research is being conducted into the effect of different vaccine combinations, the source said.
It comes after doctors and NHS trusts said planning for the autumn rollout must start now - as it will involve bigger challenges.
They said many questions needed answering, including how long immunity from the original Covid jab lasts.
The issue of whether children will be vaccinated also still remains, health leaders said.
But a Department of Health spokesman said final decisions on the booster programme would partly depend on the data from ongoing clinical trials, such as the UK's Cov-Boost trial.
That trial is testing third doses across England. The first results are expected in September and will help the Joint Committee on Vaccination and Immunisation (JCVI) take decisions on the booster programme.A government source said the UK would benefit from new vaccines from Novavax and Valneva, which are awaiting approval from the UK's medicines regulator.