World Health Organization experts have issued recommendations saying that the interval between administration of two doses of the Pfizer-BioNTech vaccine can be extended to up to six weeks.

WHO’s Strategic Advisory Group of Experts on immunization, known as SAGE, formally published guidance Friday saying that an interval of 21 to 28 days between the first and second doses is recommended.

But the U.N. health agency also noted that “a number of countries face exceptional circumstances of vaccine supply constraints combined with a high disease burden,” and some have considered postponing the administration of second doses as a way to expand the number of people initially immunized.

WHO said this “pragmatic approach” could be considered as a response to “exceptional epidemiological circumstances.” It said that countries seeking to extend the interval should make sure that vaccinated patients still have access to a second dose.

“WHO’s recommendation at present is that the interval between doses may be extended up to 42 days (6 weeks), on the basis of currently available clinical trial data,” it said, adding: “Should additional data become available on longer intervals between doses, revision of this recommendation will be considered.”

WHO also said no data is available yet on the interchangeability of the Pfizer-BioNTech vaccine with other COVID-19 vaccines. It also cited a lack of evidence about whether vaccination reduces the risk of transmission of the virus to other people.