Continuing COVID-19 Vaccination of Front-Line Workers in British Columbia with the AstraZeneca Vaccine: Benefits in the Face of Increased Risk for Prothrombotic Thrombocytopenia

Adibi, A.; Mozafarihashjin, M.; Sadatsafavi, M.

Background: On March 29th, 2021, Canada's National Advisory Committee on Immunization (NACI) recommended against using the AstraZeneca COVID-19 vaccine in younger adults pending further review of the risk for Vaccine-Induced Prothrombotic Immune Thrombocytopenia (VIPIT). As a result, the province of British Columbia halted its front-line workers vaccination program which used the AstraZeneca vaccine. The province is expected to receive an additional 246,700 doses of AstraZeneca vaccine through US and COVAX until April 11th, enough to provide the first dose of vaccine to all unvaccinated front-line workers. It is unclear whether the alternative, mRNA vaccines can be immediately made available to front-line workers. We evaluated the harms and benefits of delaying vaccination of front-line workers in BC. Methods: We reviewed the latest available evidence and used compartmental modelling to *1)* compare the expected number of deaths due to COVID-19 and VIPIT under the scenarios of immediately continuing vaccination of front-line workers with the AstraZeneca vaccine or delaying it in favour of mRNA vaccines, and *2)* compare the individual mortality risk of immediately receiving the AstraZeneca vaccine with waiting to receive an mRNA vaccine later for different age groups. Results: We estimate that if British Columbia continues the front-line worker vaccination program with the AstraZeneca vaccine, we expect to see approximately 27,000 fewer cases of COVID-19, 500 fewer hospitalizations, 80 fewer COVID-related deaths, and 1,400 fewer cases of Long COVID from April 1st to October 1st, 2021, for an expected number of VIPIT-related deaths of 0.674 [95% CI 0.414-0.997]. In the same period and in areas of high transmission, the projected excess risk of mortality due to COVID-19 and VIPIT was significantly higher in the delayed vaccination with the mRNA vaccine scenario (3.23 to 4.44 times higher risk) than that of immediate vaccination with the AstraZeneca vaccine for those between 30 and 69 years of age. For those under 30, immediate vaccination with the AstraZeneca vaccine posed a higher risk than delayed vaccination with an mRNA vaccine. Conclusions: The benefits of immediately continuing immunization of front-line workers with the AstraZeneca vaccine far outweigh the risk both at a societal level and at an individual risk level for those over 40, and those over 30 in high-risk areas.