Breakthrough Infections After COVID Vaccination

July 30, 2021

On Wednesday, the New England Journal of Medicine published a study evaluating data obtained from fully vaccinated healthcare workers at Israel’s largest hospital. The study focused on breakthrough Covid-19 infections, defined as detection of the SARS-CoV-2 virus via a RT-PCR test 11 or more days after receiving the second dose of the Pfizer vaccine. Of 11,453 vaccinated healthcare workers, 1,497 were tested for COVID-19 during the study, and 39 breakthrough infections were documented.

Of all the workers with breakthrough infection, 26 (67%) had mild symptoms at some stage, 13 were asymptomatic, and none required hospitalization.
The investigators of the study obtained antibody titers from 22 of the employees from before they were infected with SARS-CoV-2. Serological samples from breakthrough cases were matched with 4-5 samples from uninfected controls based on a series of factors (including age, sex, immunosuppression status, and the amount of time between receiving the second dose of the Pfizer vaccine to getting tested). The breakthrough cases showed lower levels of neutralizing antibodies. This is an important first step in discovering the threshold of neutralizing antibody protection that is needed to combat breakthrough infections.