Over time, the number of SARS-CoV-2 antibodies falls in both previously infected and vaccinated people, but the performance of antibodies improves only after previous infection and not vaccination.

This groundbreaking research, led by Dr. Carmit Cohen of Israel’s Sheba Medical Center, will be presented at the European Congress of Clinical Microbiology and Infectious Diseases in April.

Cohen’s finding may explain why previously infected patients appear to be better protected against a new Covid infection than those who have only been vaccinated.

She and her colleagues analyzed antibody-induced immune response in 130 recovered individuals for up to a year and compared it to 402 matched individuals who were double-vaccinated with the Pfizer vaccine but never had Covid-19.

The researchers found that the numbers of antibodies a month after vaccination were higher than those in the Covid-19 recovered patients. However, these numbers declined more steeply in the vaccinated group.

While the avidity (antibody performance quality) index was higher in vaccinated individuals than in recovered patients initially, avidity did not significantly change over time in vaccinated individuals but increased gradually in recovered patients.

This could explain why double-vaccinated individuals who never contracted Covid-19 are more likely to experience infection after six months.

The study also found that, contrary to expectations, antibodies of recovered patients with a body mass index of 30 or higher (in the obese range) were higher at all time points when compared with those with a BMI under 30 (overweight to normal weight range). The obese people who had been previously infected were therefore better protected against future infection.

Of all recovered patients, 42 (36%) experienced long Covid including mental health (5%), neurological (9%), cardiovascular (5%) and respiratory (31%) symptoms.

The authors conclude: “While the number of antibodies decrease with time in both Covid-19 recovered patients and vaccinated individuals, the quality of antibodies increases following infection but not after vaccination. These results provide specific characteristics of the immune response that may explain the differential protection against Covid-19 in previously infected and vaccinated individuals.”