— Study shows importance of vaccination to prevent severe disease, even among previously infected
by Molly Walker
n people hospitalized for COVID-like respiratory illnesses, unvaccinated adults who previously had COVID-19 were over five times more likely to test positive for SARS-CoV-2 compared with recipients of Pfizer or Moderna vaccine, researchers found.
Among more than 7,000 people hospitalized for typical COVID-like symptoms, 8.7% of unvaccinated adults with a prior infection at least 3 months before tested positive for SARS-CoV-2 again, as compared to 5.1% of the fully vaccinated patients without a prior history of infection, for an adjusted odds ratio (OR) of 5.49 (95% CI 2.75-10.99) after factoring in local positivity rates, age, and numerous other factors, reported Catherine Bozio, PhD, of the CDC, and colleagues, writing in an early edition of the Morbidity and Mortality Weekly Report.
Proponents of prior infection have traded salvos with those supporting vaccination over the past few months, as dueling reports emerged. First, there was the study in Kentucky in early August that found COVID vaccination doubled protection among previously infected individuals versus those who had not been vaccinated. Then a preprint from Israel at the end of August found prior infection was superior to vaccination against the Delta variant.
Other studies cast doubt on the effectiveness of vaccination, such as the study from late July that found that three-quarters of individuals in a COVID outbreak on Cape Cod were previously vaccinated, though most of the cases were mild.
CDC Director Rochelle Walensky, MD, weighed in on the new evidence on Friday from more than 7,348 people across nine states.
“We now have additional evidence that reaffirms the importance of COVID-19 vaccines, even if you have had prior infection,” she said in a statement. “This study adds more to the body of knowledge demonstrating the protection of vaccines against severe disease from COVID-19.”
Bozio’s team examined data on hospitalizations from January 1 to September 2, 2021 from the VISION Network, a group of 187 hospitals in nine U.S. states. Participants were adults ages 18 and up with COVID-19 molecular testing 14 days to before 72 hours prior to admission with a “COVID-like illness discharge diagnosis,” and who had been tested at least once since February 1, 2020.
The researchers noted that adults were considered “unvaccinated” with a previous infection if there was no record of COVID-19 vaccine doses received and the most recent positive test for SARS-CoV-2 occurred more than 90 days before hospitalization. Adults were considered vaccinated if they had received the second Pfizer or Moderna dose at least 14 days before hospitalization. Partially vaccinated patients, those who received Johnson & Johnson vaccine, and those with a positive test result 14-89 days before hospitalization were excluded.
Overall, 201,269 hospitalizations were identified, with 139,655 patients hospitalized after vaccines were eligible for their particular age group. Molecular testing was performed for 94,264 patients with hospitalizations for COVID-like illness.
There were 1,020 hospitalizations among previously infected and unvaccinated adults, and 6,328 hospitalizations among previously vaccinated adults who met the study criteria.
The vaccinated group tended to be far older than the unvaccinated cohort:
- 65-74 years: 28% vs 20%
- 75-84 years: 32% vs 17%
- 85 and up: 18% vs 8%
While more unvaccinated adults were ages 18-49 versus vaccinated adults (31% vs 9%, respectively), Black (10% vs 7%), and Hispanic (19% vs 12%).
“Secondary analyses that did not adjust for time since infection or vaccination or adjusted time since infection or vaccination differently as well as before and during Delta variant predominance produced similar results,” the authors wrote.
They concluded that “all eligible persons should be vaccinated against COVID-19 as soon as possible, including unvaccinated persons previously infected with SARS-CoV-2.”