The European Medicines Agency (EMA) says people who got the one-shot Janssen vaccine can get a booster two months afterwards.
It says a booster dose may be considered "at least two months after the first dose in people aged 18 years and above".
The recommendation follows data showing that a booster dose of Janssen - also called Johnson & Johnson - given at least two months after the first dose led to a rise in antibodies against the virus.
The EMA also says a Janssen booster can be given after two doses of the Pfizer or Moderna vaccines.
In a statement, it says: "At national level, public health bodies may issue official recommendations on the use of booster doses, either following one dose of COVID-19 vaccine Janssen or two doses of the mRNA vaccines, taking into account the local epidemiological situation, availability of vaccines, and emerging effectiveness and the limited safety data for the booster dose."
The Government has recently reduced the waiting period between vaccination and booster doses to three months.
People vaccinated with anything but Janssen had to wait a minimum of five months after being deemed fully vaccinated before getting their booster.
The waiting period for people who received the Janssen vaccine had already been reduced to three months.
It is not yet clear if the Government will reduce this window further.