Vaccine dose interval extended from 28 days – why?

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Why has the interval between vaccine doses been extended from the Pfizer and Moderna product monograph intervals of 21 or 28 days?

Both Pfizer-BioNTech and Moderna vaccines are given as a two-dose series. The minimum interval between Pfizer doses is 21 days, for the Moderna vaccine, it’s 28 days, but a longer interval is not expected to reduce effectiveness given published data to date, and experience with all other vaccines given in series. Published studies of both vaccines have shown that strong protection is achieved even after the first dose (>90% efficacy beginning from 14 days after dose 1 and prior to dose 2). A longer interval in administering the second dose will not require starting the series again. (NACI: Recommendations on the use of COVID-19 vaccines)

The Joint Committee on Vaccination and Immunization in the UK published a recommendation supporting an interval of up to 12 weeks between two doses of COVI-19 vaccines (JCVI: advice on priority groups for COVID-19 vaccination). The Province of Quebec has also announced a decision to defer dose 2 until a number of priority populations have been immunized. Similar recommendations are expected in other jurisdictions.

Last week, Dr. Bonnie Henry, Provincial Health Officer, explained the decision to extend the interval between doses for people in B.C. From looking at data, modelling from BC Centre for Disease Control, and using the COVID-19 Ethical Decision-Making Framework, it has been determined that administering the first doses of vaccine to as many people as possible in the priority groups in the early weeks of the campaign, when doses are limited, will protect the greatest number of at-risk people overall in the shortest possible time, and will have the greatest impact on reducing mortality, severe disease and hospitalizations. Based on this direction, we will be planning to provide the second dose of both the Pfizer and Moderna vaccines at approximately 35 days after the first dose.

Public Health is tracking who has received vaccine, which kind was administered, and when people will be invited to receive their second dose, but you can help by making a note of your own dates and details, and by being ready when it’s time for your second vaccination.

*From VCH_Information on does interval for COIVD-19 vaccine.pdf