Hospitalizations after breakthrough COVID-19 cases 'extremely uncommon': study

Hospitalizations among adults with breakthrough cases of COVID-19 are 'extremely uncommon,' a new study suggests.

According to Mayo Clinic research, published in this month's Clinical Infectious Diseases, fewer than 1 in 1,000 people who have been vaccinated against COVID-19 or have been previously infected with COVID-19 were hospitalized after a breakthrough infection.

"In the general primary care patient population, those who have been vaccinated have very low risk of subsequent hospitalization for breakthrough COVID-19," Dr. Benjamin Pollock, the study's lead author, said in a news release. "Our study shows that while it can and does happen, that these occurrences are extremely uncommon."

Researchers studied more than 100,000 people who were 18 years old or older over the course of 21 months. Of those patients, 69 were later hospitalized after a breakthrough infection of COVID-19.

For vaccinated patients, the rate of hospitalization was 0.06 per cent, or six people in 10,000. For previously infected but unvaccinated people, the rate was 0.03 per cent, or three people in 10,000.

For those who had previously been vaccinated and infected, the rate was 0.01 per cent, or one person out of 10,000. Researchers said the small difference in infection rates between the three groups was "statistically insignificant."

"We found these results to be in line with previous studies, although the interpretation shouldn't necessarily be that natural immunity provides the same protection as vaccination," Pollock said. "Rather, this study found that among our primary care population, both natural immunity and vaccine immunity appeared to lead to very low rates of breakthrough hospitalizations."

Given the risks associated with becoming infected with COVID-19, Dr. Aaron Tande, a Mayo Clinic infectious diseases physician and a co-author of the study, said vaccination is still the safest and most effective way to protect against serious illness.

"Because it's impossible to tell in advance how severe a first infection may be, or who among vulnerable populations the virus may spread to, waiting for natural immunity is a gamble and not a safe alternative," Tande said.

The study's authors said that their analysis was limited by the lack of adjustment for the different variants of COVID-19, such as the highly-transmissible Omicron variant or its subvariant BA.2, but note that other studies about natural and vaccine immunity have produced similar results.

"While breakthrough infections are increasingly reported, infections that result in hospitalization are rare in those with either type of immunity," researchers said. "Primary care physicians should continue to promote COVID-19 vaccination as an evidence-based method of limiting the risks of future COVID-19 hospitalization."