Although not 100% effective vaccines against coronavirus protect against severe forms of the disease, and some of these vaccines can also reduce the risk of transmission. A U.S. study tracked infections among vaccinated health care workers at two universities to determine when the risk of becoming infected is highest.
Covid-19 vaccines are designed to minimize severe forms of the disease. Their ability to prevent coronavirus infection was not part of the clinical trials conducted before they were approved. But scientists have since done that work, particularly for the messenger RNA vaccines from Pfizer and Moderna. According to the study, these two vaccines reduce the risk of infection by 80% after the first injection and 90% after the second.
Doctors in California have also been looking at this issue. They have tracked cases of coronavirus infection among nurses at two universities in Los Angeles and San Diego. They shared their work in The New England Journal of Medicine.
The period between doses, a high-risk period.
The follow-up of health professionals took place between Dec. 16, 2020, and Feb. 9, 2021, a period when nasopharyngeal screening tests were available for this population particularly exposed to the virus, including those without symptoms. A total of 14,604 people were tested, regardless of whether they had received one or two doses of Pfizer’s vaccine.
The results showed that most of those infected became infected within 15 days of the first vaccine injection. This is when protection from the vaccine is weakest. The immune system has not yet developed specific antibodies against the coronavirus. A total of 342 people were infected between the two doses and only 37 after the second dose.
A risk of infection after vaccination of about 1%
In the University of Los Angeles cohort, the absolute risk of becoming infected with SARS-CoV-2 after Pfizer vaccination was 1.19%, and in the University of San Diego cohort, 0.97%. Here, the incidence of coronavirus after vaccination is higher than in the clinical trial (100 cases of covid-19 in 21,669 participants) because health care workers are much more exposed to the virus than participants in the clinical trial.
Nevertheless, both studies show the same trend. Most infections in vaccinated individuals occurred between the first and second doses. During this time, observing the recommended precautions and wearing masks is more necessary than ever.