A new study out of Israel offers more insight into COVID-19 breakthrough cases, showing that many cases occur in people with lower levels of antibodies.
A breakthrough case is defined as someone who has been infected with the coronavirus despite being fully vaccinated.
Israel’s findings were reported in The New England Journal of Medicine.
Researchers studied 1, 497 fully vaccinated health care workers at Sheba Medical Center out of 11,453 fully vaccinated workers. All had been vaccinated with the Pfizer-BioNTech COVID-19 vaccine.
They were tested for breakthrough infections from December 19, 2020, to April 28, 2021, due to a known exposure with someone who had COVID-19 or possible symptoms of the virus.
Thirty-nine workers had a confirmed breakthrough case, bringing the infection rate to 0.4%, Many had mild to no symptoms and none required hospitalization. The most common symptoms included respiratory congestion, muscle aches (myalgia), and loss of smell or taste. Most didn’t develop a fever. Some reported having long COVID-19 symptoms such as prolonged loss of smell, persistent cough, weakness, and fatigue.
The average age of workers with breakthrough cases was 42 and only one person was known to have a weakened immune system.
According to the study, 22 out of the 39 breakthrough cases provided results from an antibody test taken the week before their COVID-19 diagnosis. The results showed that those with a breakthrough infection had consistently lower levels of antibodies circulating in their bloodstream. On the other hand, those with high levels of antibodies were associated with greater protection and lower infectivity.
But researchers pointed out they’re not sure if waning immunity from the vaccine was also a factor in the infections. They also noted that the study was conducted before the more-transmissible delta variant sparked a surge in cases.
"Overall, these findings provide more reassurance that these vaccines are extremely effective," Dr. Francis Collins said in a National Institutes of Health blog. "Breakthrough infections, while they can and do occur, are a relatively uncommon event."
This story was reported from Los Angeles.