TIMING - Timing of second dose of SARS-CoV2 mRNA vaccine on immunologic and neutralizing antibody responses generated in healthcare workers [Funder: PHRI]

This real-world study is addressing the marked and unexpected reduction in the supply of the mRNA vaccines to Canada, as of January 2021, which has led to a policy change of delaying the second dose of the Pfizer vaccine to healthcare workers (HCW) up to 35 to 42 days. There is little evidence to support this policy, which could be harmful if immunity diminishes to low levels, leaving HCWs at risk of infection before the delayed second dose. Furthermore, there is no data to inform how dosing schedules may impact on the secondary immune responses and long term immunity. However, this policy represents a potential natural experiment and an opportunity to study the impact of dosing schedule on vaccine immunogenicity as it relates to short and long term immunity. Since the sequence and timing interval of dosing to HCWs are largely randomly generated; the variation in time interval between the first and second vaccine doses is unbiased and unaffected by any factors other than by chance.