Japan could mix AstraZeneca PLC’s COVID-19 shots with those developed by other companies in order to speed up its vaccination effort, according to the minister in charge of the country’s rollout.

The idea would be to combine the dose with one from Pfizer Inc. and BioNTech SE or Moderna Inc., Taro Kono said Sunday on a Fuji TV news program. Since AstraZeneca recommends eight weeks between its two shots, it’s likely that time could be shortened if combined, Kono added.

Studies on mixing COVID-19 vaccines so far show that it could result in an equal if not stronger immune response. But it’s not clear which combination could give the best protection that will last the longest, and there’s some evidence that mixing could cause worse side effects. Countries like Canada, Germany and France allow mix-and-match vaccines, but the U.S. has warned against doing so.