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Cosmos Study

Getting COVID-19 Twice: Reinfections 44% More Likely Among the Unvaccinated

Abstract: Unvaccinated patients are more likely than vaccinated patients to get COVID-19 twice.
December 13, 2021
Dual-Team Study
Team A:Karina Rohrer-Meck, BSN, RNEric BarkleyLindsay Lin, PhD
Team B:Brad Fox, MDJoe McNitt

To examine how vaccines affect immunity for people who have already had COVID-19, we compared reinfection rates of COVID-19 in vaccinated and unvaccinated patients and found that unvaccinated patients were significantly more likely to be reinfected. To best determine whether a patient was vaccinated or unvaccinated, our study population included those with a face-to-face encounter where vaccination status could be verified.

While the primary benefit of vaccination is to decrease the severity of infection1, it also appears to have a modest benefit in preventing reinfection. This finding was based on an average weekly reinfection rate of 1.6 out of 10,000 for partially or fully vaccinated patients and 2.3 out of 10,000 for unvaccinated patients. This equates to one prevented reinfection for every 275 patients vaccinated over a period of 12 months.

Figure 1
Reinfection Rates Between Vaccinated and Unvaccinated Patients
Reinfection Rates Between Vaccinated and Unvaccinated Patients
Figure 1. The lines above show the average weekly reinfection rates per 10,000 by vaccination status: unvaccinated (orange) and fully vaccinated (green). Unvaccinated patients had consistently higher rates of reinfection.

Among patients who were reinfected, hospitalization rates were similar for both vaccinated and unvaccinated patients, with about one in five reinfected patients admitted to the hospital. These findings are consistent with other research demonstrating a decreased risk of COVID reinfection in vaccinated patients2.

These data come from Cosmos, a HIPAA-defined Limited Data Set of more than 120 million patients from 141 Epic organizations including 832 hospitals and 13,421 clinics, serving patients in all 50 states. This study was completed by two teams, each comprised of a clinician and research scientists who worked independently. The two teams came to similar conclusions.

  1. Centers for Disease Control and Prevention. COVID-19 Vaccines Work. https://www.cdc.gov/coronavirus/2019-ncov/vaccines/effectiveness/work.html. Accessed on: December 3, 2021.
  2. Cavanaugh AM, Spicer KB, Thoroughman D, Glick C, Winter K. Reduced Risk of Reinfection with SARS-CoV-2 After COVID-19 Vaccination — Kentucky, May–June 2021. MMWR Morb Mortal Wkly Rep 2021;70:1081-1083. DOI: http://dx.doi.org/10.15585/mmwr.mm7032e1external icon