Coronavirus vaccine dosing intervals: What countries are doing around the world
As the world races to contain rising COVID-19 infections fuelled by new coronavirus variants, some countries are seeking to counter low vaccine supplies with dosing patterns or volumes that stray from how the shots were tested in clinical trials.
There are differences over the merits of alternative dosing strategies, with some arguing the urgency of the pandemic requires flexibility, while others oppose abandoning data-driven approaches for the sake of expediency.
Here is a list of what countries are doing:
* The U.S. Food and Drug Administration has said that altering the authorised dosing or interval schedules of COVID-19 vaccines is premature and not supported by science, while acknowledging that such questions may be "reasonable" to consider. It is calling for 21 days between doses of Pfizer and BioNTech's shot, and 28 days for Moderna's vaccine.
* In Britain, regulators have said shots can be administered up to 12 weeks apart for vaccines with emergency approval from AstraZeneca and its partner Oxford University, as well as Pfizer/BioNTech.
During trials of the Oxford/AstraZeneca shot, the median dosing interval varied between countries, with 10 weeks in British trials and six weeks in Brazil. But Pfizer says its shot has only been properly evaluated with a 21-day interval between doses and there is no data to show that protection is sustained beyond 21 days after the first dose.
Britain has also said it will allow people to be given shots of different COVID-19 vaccines on rare occasions, despite acknowledging there "is no evidence on the interchangeability of the COVID-19 vaccines".
* The European Medicines Agency (EMA) has said there should be a maximum interval of 42 days between the first and the second shot of the Pfizer-BioNTech vaccine that it has approved, based on a study where shots were given 19 to 42 days apart with full protection achieved seven days after the booster.
* Denmark has approved an interval of up to six weeks between the first and second Pfizer/BioNTech shots, in line with EU guidance. It has also said the original guidelines of waiting only three to four weeks should be followed whenever possible.
* Germany is considering whether to allow a delay, seeking an independent vaccination commission's guidance on whether to push a second shot beyond the current 42-day maximum limit amid frustration and criticism within the country over what some see as a sluggish launch.
* The health ministry in Belgium says it is planning to follow EMA recommendations for COVID-19 vaccine doses. Ultimately, regional authorities in Belgium typically decide on vaccination plans, but there are no indications there will be differentiated approaches across the nation, a spokeswoman said.
* The Dutch health minister says the nation is considering waiting longer with a second shot.
* The Irish Immunisation Advisory Committee has recommended continued administering of two doses of the Pfizer-BioNTech vaccine in the period indicated by its manufacturers.
* The central health authority will continue to recommend administering two doses of the Pfizer-BioNTech vaccine in the period indicated by its manufacturers, Health Minister Salvador Illa said.
* Switzerland, which as a non-EU country has its own drugs regulator called swissmedic that has approved Pfizer/BioNTech's shot, is not planning at this time to deviate from the label of vaccines in order to stretch doses, the Swiss Federal Health Ministry said.
* Regulators who approved the Oxford/AstraZeneca vaccine, to be made in the country by the Serum Institute of India, have recommended it be administered eight to 10 weeks apart.