Fourth-dose Covid booster vaccines increase protection against Covid-19, particularly in people aged over 70, according to a UK trial.
But scientists say any short-term protection against infection is likely to fall away quickly.
The UK rolled out fourth doses to over-75s and the most vulnerable in April.
A larger group of people may be offered a booster in the autumn, but any decision will be based on advice from the UK's vaccine committee.
It is likely to look at whether new worrying variants are spreading, and Covid pressure on hospitals.
Some countries, such as Israel and Germany, have already started offering all adults a fourth dose.
Health and Social Care Secretary Sajid Javid said the study findings were "further evidence underlining the importance of people coming forward for their booster as soon as they are eligible".