Competitors in next year's Boston Marathon must provide proof of a COVID-19 vaccine, organisers said on Tuesday, adding to a growing list of major sport events where inoculation is mandatory for participation.

The race, the world's oldest annual marathon, will return to its usual April date after pushing the 2021 edition to October due to the pandemic and will allow a field of 30,000 after capping the participants to 20,000 this year.

The race did not take place in 2020 due to the pandemic.

"We had a 93 per cent vaccination rate among our 125th Boston Marathon participants and want to do our part to continue to mitigate the spread of COVID-19 as we continue our return to racing," Boston Athletic Association President and CEO Tom Grilk said in a statement.

Participants in last month's race were required to show proof of a vaccine or a negative test, similar to the restrictions in place for Sunday's New York City Marathon.

The 2022 Boston Marathon will take place on April 18.