Severe COVID-19 Cases Very Rare Among Vaccinated, Previously Infected

Reports of breakthrough cases were alarming for those who thought they had protection against COVID-19, either through vaccination or previous infection. A new study suggests that those people had less to worry about than they feared, finding that the risk of getting sick enough from SARS-CoV-2 infection to require hospitalization was extremely low for those cohorts. Here is more information.

ROCHESTER, MI — The chance of ending up hospitalized with COVID-19 is exceptionally low for people who have immunity, either through vaccination or prior infection, according to a new study.

In fact, the report in Clinical Infectious Diseases points out that fewer than 1 in 1,000 people who have been vaccinated or previously infected with COVID-19 were hospitalized with a new 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," explained lead author Benjamin Pollock, PhD., a researcher in the Mayo Clinic Robert D. and Patricia E. Kern Center for the Science of Health Care Delivery. "Our study shows that while it can and does happen, that these occurrences are extremely uncommon."

Researchers used a longitudinal study of 106,349 adult primary care patients at Mayo Clinic in Rochester who tested positive for COVID-19, and/or were vaccinated for COVID-19. They determined that only 69 of those patients were hospitalized with a breakthrough COVID-19 infection.

Specifically, the hospitalization rate was:

  • 0.06%, or 6 in 10,000 for vaccinated patients.
  • 0.03%, or 3 in 10,000, in previously infected but unvaccinated people.
  • 0.01%, or 1 in 10,000, among those who were both vaccinated and infected previously.

"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 advised. “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."

The study specifically focused on breakthrough cases that resulted in hospitalization and did not include information on immunity after infection and vaccination rates among mild or asymptomatic breakthrough cases.

"We know that vaccination remains the safest route to protection from COVID-19 infection and severe disease," added co-author Aaron Tande, MD, a Mayo Clinic infectious diseases physician. "I explain to my patients that a COVID-19 vaccine provides additional protection, even if they have been previously infected. For those who have not been infected, vaccination remains the safest and most reliable route of protection."

Tande emphasized, "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.”

The authors point out that “large, real-world observational studies are still needed to determine the comparative effectiveness of natural immunity versus vaccination in preventing COVID-19 hospitalization. While breakthrough infections are increasingly reported, infections that result in hospitalization are rare in those with either type of immunity. Primary care physicians should continue to promote COVID-19 vaccination as an evidence-based method of limiting the risks of future COVID-19 hospitalization.”

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