In one report, just 0.4 percent of healthcare workers in Israel who received both shots of the COVID-19 immunization later tested positive compared to 7.2 percent of unvaccinated employees.
This means healthcare workers who received both doses were 18 times less likely to fall ill.
Meanwhile, in the second study, 1.8 percent of fully vaccinated frontline staff at a hospital in Tennessee contracted the virus in comparison with 8.5 percent of unvaccinated workers, putting the latter group at a 4.7 times greater risk of infection.
The results are in line with the 95 percent efficacy Pfizer and BioNTech reported from their late-stage clinical trial in December 2020 that led to emergency use authorization from the Food and Drug Administration (FDA).
Because vaccinated people were not only less likely to get sick from coronavirus, but to even get an asymptomatic infection, the findings are also a sign that the vaccine may prevent transmission between close contacts.
At a hospital in Tel Aviv, Israel, 5,517 healthcare workers, or 82.2% (light blue line), received two doses between December and February and 757, 11.3% (yellow line) were not vaccinated. Of them, 0.4% of the fully vaccinated later tested positive for Covid compared to 7.2% of unvaccinated people
In a second study at a hospital in Tennessee, 2,776 employees, 53.2%, were fully vaccinated 2,165 staff members, 41.5%, were unvaccinated. Of those, 1.8% of the fully vaccinated later tested positive for Covid (yellow line) compared to 8.5% of the unvaccinated workers (blue line)
For the study, published in the Journal of the American Medical Association (JAMA), the team from the Tel Aviv Sourasky Medical Center collected data from 6,710 of their employees.
Between December 20, 2020, and February 25, 2021, 5,953, or 88.7 percent, received at least one dose and 5,517, or 82.2 percent, received two doses.
The remaining 757, 11.3 percent, were not vaccinated.
Workers were considered fully vaccinated once it was seven days or later after receiving their final dose.
Of the fully vaccinated employees, 27 of the 5,517, or 0.4 percent, later tested positive for the virus, as did 55 of the 757, or 7.2 percent, of the unvaccinated staff.
In the Israel study, just 8 fully vaccinated people (solid blue line) were symptomatic and 19 were asymptomatic (dotted blue line) compared to 38 symptomatic unvaccinated workers (solid yellow line) and 17 asymptomatic workers (dotted yellow line)
Researchers also compared rates of symptomatic and asymptomatic illness.
Eight of the fully vaccinated people who tested positive had symptoms such as cough, fever and shortness of breath compared to 38 of the unvaccinated workers.
The team says this shows that not only is the vaccine effective at blocking illness but, if a person does get sick, they are very unlikely to experience symptoms.
They determined that Pfizer’s shot was 97 percent effective at preventing symptomatic illness and 86 percent effective at preventing asymptomatic infection.
‘In this retrospective cohort study of regularly screened health care workers, vaccination with 2 doses of the [Pfizer] vaccine was associated with significantly lower incidence rates for both symptomatic and asymptomatic SARS-CoV-2 infection,’ the authors wrote.
In the Tennessee study, 56.9% of fully vaccinated people were asymptomatic (left, yellow line) and 43.1% were symptomatic (right, yellow line) compared to 42.7% of unimmunized workers who were asymptomatic (left, blue line) and 57.3% who ere symptomatic (right, blue line)
For the second study, also published in JAMA, the team from St Jude Children’s Research Hospital, Memphis, Tennessee, looked at data from 5,217 employees.
Between December 17, 2020 and March 20, 2021, 3,052, 58.5 percent, received at least one dose and 2,776, or 53.2 percent, received both doses.
The remaining 2,165 staff members, or 41.5 percent, were unvaccinated.
Nasal swab tests showed that at least 10 days after the second dose, 1.8 percent – 51 out of 2,776 – tested positive compared to 8.5 percent – 185 out of 2,165 people.
Similarly to the first study, the majority of fully vaccinated people were asymptomatic at 56.9 percent compared to 42.7 percent of unimmunized workers.
‘Unvaccinated employees had higher cumulative incidence of a positive test result than vaccinated employees, and higher incidences of positive test results via asymptomatic screening, for symptoms, or for known exposure,’ the authors wrote.