The Imvanex jab can now be given just below the top layer of skin, or intradermally, instead of fully below the skin, or subcutaneously, as it currently is, the European Medicines Agency (EMA) said.
The new technique uses just one fifth of the amount of vaccine but produced similar levels of antibodies, although there was a greater risk of skin irritation, the Amsterdam-based watchdog said in a statement.
"National authorities may decide as a temporary measure to use Imvanex as an intradermal injection at a lower dose to protect at-risk individuals during the current monkeypox outbreak while supply of the vaccine remains limited," the EMA said.
EU Commissioner Stella Kyriakides said the decision was "extremely important as it allows the vaccination of five times as many people with the vaccine supply we have".
"This ensures greater access to vaccination for citizens at risk and healthcare workers," she said in a statement.
The ability to protext against the virus with less may help Africa deal with its monkeypox problem by extending the reach of available vaccine supplies on the continent.