A study conducted by the Institut national de santé publique du Québec (INSPQ) among health workers reveals a high rate of effectiveness of messenger RNA vaccines against infections caused by SARS-CoV-2. The Pfizer and Moderna vaccines, which were analyzed because they were predominantly offered to this population, are 70.9% effective in preventing any SARS-CoV-2 infection after a first dose and 86.9% after the second dose. For symptomatic infections, the efficacy is 73.4% after one dose and 94.2% after two doses. Finally, the efficacy in preventing hospitalizations is 97.9% after the administration of a single dose of vaccine and 100% after two doses.
These data are reassuring for the immunization prospects of the general population and in particular for the protection of healthcare workers, who, by the nature of their work, constitute the group most exposed to the virus. The health workers who took part in the study are part of the staff paid by public or private health establishments under agreement. Participants do not include workers in clinics, pharmacies, private seniors’ residences, etc., nor doctors and medical residents.
Stable vaccine efficacy for 16 weeks after the first dose
It is interesting to note that, overall, the vaccine efficacy remained stable for the period between 2 and 16 weeks following the injection of the first dose. This finding shows that the decision of the Government of Quebec to delay the administration of the 2e dose with the aim of providing some immunity to a larger proportion of the population has proved to be appropriate.
Good vaccine efficacy against the Alpha variant
The emergence of worrying variants at the start of the year raised many questions about vaccine efficacy in the field. In Quebec, the Alpha variant which first appeared in the United Kingdom is the one which predominated during the period analyzed. The study shows good efficacy against this variant, which does not decrease over time, but which is nevertheless 10% lower than that against other strains of SARS-CoV-2.
Immunization status of health workers
As of May 22, 2021, 81.9% of health workers in public or private institutions under agreement had received at least one dose of the vaccine against COVID-19 and 34.6% had received two doses. Note that the vaccine efficacy is calculated from 14 days or more after a single dose of vaccine and 7 days or more after two doses, with an interval of at least 3 weeks between doses according to the manufacturer’s instructions.