Efficacy of two-dose SARS-CoV-2 vaccine against infection and hospitalization in Canada

In December 2020, two mRNA vaccines were approved for use against Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) in Canada, BNT162b2 (Pfizer-BioNTech) and mRNA-1273 (Moderna). The chimpanzee adenoviral vector vaccine (ChAdOx1) (AstraZeneca) was also authorized in February 2021. The two mRNA-based vaccines were to be administered in two doses, with intervals of three weeks for the BNT162b2 vaccine and four weeks. for mRNA-1273 vaccine. The ChAdOx1 vaccine was to be administered in two doses with an interval between doses of 4 to 12 weeks.

To study: Efficacy of two-dose SARS-CoV-2 vaccine with mixed schedules and extended dosing intervals: negative test design studies from British Columbia and Quebec, Canada. Image Credit: JLStock / Shutterstock

A second dose deferral approach was implemented in March 2021 by the National Advisory Committee on Immunization (NACI) of Canada, which recommended an extension of the interval between the first and second dose of vaccine of up to up to 16 weeks. Healthcare workers and residents of long-term care facilities (LTCFs) have been prioritized for the first doses of SARS-CoV-2 vaccine in British Columbia (BC) and Quebec. In March 2021, vaccination began among adults living in the community, gradually progressing to younger age groups.

In this study, authors from various Canadian institutes report the efficacy of the two-dose (VE) vaccine against infection and hospitalization, which includes Delta variant infection of concern (VOC) in British Columbia and the United States. Quebec.

A pre-printed version of this study, which is yet to be peer reviewed, is available on the website medRxiv* preprint server.

The study

The authors collected 380,532 specimens for testing for VE in British Columbia, of which 27,439 were positive cases and 1,582 were from hospitalized cases. A total of 854,915 cases were collected in Quebec, including 17,234 positive cases in the test and 878 hospitalized cases. The Delta variant accounted for 91% of cases in British Columbia and 85% of cases in Quebec.

The VE with the two doses of mRNA vaccines was 90% in British Columbia and 88% in Quebec. The VE of ChAdOx1 vaccine was significantly lower in the two-dose recipients, with 71% in British Columbia and 73% in Quebec. However, VE was significantly improved in people who received a mixed schedule of mRNA and ChAdOx1 vaccine, with 90% in British Columbia and 87% in Quebec.

VE against hospitalization with two doses of mRNA vaccines was 98% in British Columbia and 97% in Quebec and was similar for the ChAdOx1 vaccine with 94% in British Columbia and 94% in Quebec . Similar levels of VE were also observed in recipients of mixed mRNA doses or mixed doses of ChAdOx1 / mRNA. The VE for both types of Delta variant vaccine was almost identical to the overall analysis and was similar among other VOCs.

The two-dose VE against hospitalization with mRNA-based vaccines in British Columbia and Quebec remained ≥ 95% seven months after vaccination. EV against infection with mRNA vaccines peaked 2-3 weeks after vaccination at 90% but remained at 80% eight months after vaccination. Recipients of the two-dose ChAdOx1 homologous vaccine exhibited a 70% VE, which was maintained for at least four months after vaccination. None of the results collected indicated a greater drop in VE compared to the Delta variant.

The VE of mRNA-based vaccines was improved when there were longer intervals between the first and second dose. With the proposed interval specified by the manufacturers of 3 to 4 weeks, the VE in British Columbia was 85% and 79% in Quebec. VE against hospitalizations was higher with an interval of 7 to 8 weeks (99%) compared to an interval of 3 to 4 weeks (93%).

Due to a shorter interval between vaccines, meaning there would have been a longer delay from the second dose when samples were collected, the authors explored the simultaneous stratified VE for period effects. . When the second dose was given ≥ 7 weeks as opposed to 3 to 4 weeks after the first, the increase in VE of 5 to 10% was maintained at all times since the second dose.

Implications

The results of this study demonstrated that two doses of mRNA and / or ChAdOx1 vaccines provide strong protection against hospitalization with no minimal signs of decline 5-7 months after vaccination in adults living in the community. These vaccines have also been shown to provide long-lasting protection against VOCs, including the Delta variant.

There was a slight decrease in VE against infection seen from the peak of 90%, but it remained at about 80% seven months after vaccination. VE improved when the interval between the first and second dose was lengthened. These results support mixed SARS-CoV-2 immunization schedules and longer intervals between the first and second doses, each of which could improve immunization coverage globally.

*Important Notice

medRxiv publishes preliminary scientific reports which are not peer reviewed and, therefore, should not be considered conclusive, guide clinical practice / health-related behavior, or treated as established information.