Large study determines number of adults who need to be vaccinated to prevent COVID-19 hospitalizations and ED visits

The study found that the number of patients needed to be vaccinated to prevent one COVID-19-associated hospitalization was higher than the number needed to vaccinate to prevent one COVID-19 associated ED visit, reflecting differences in outcome severity. These numbers were dependent on patient risk factors as well as local disease incidence.

The number needed to be vaccinated to prevent one COVID-19-associated hospitalization ranged from 44 to 615 (median was 205) individuals and was lower for adults aged 65 years or older and for those with underlying medical conditions. The number needed to be vaccinated decreased as the population became older because older individuals are more susceptible to the adverse effects of the virus and, therefore, the vaccine provides greater protection.

The number of patients needed to be vaccinated to prevent COVID-19-associated ED visits showed a different pattern because vaccines were more effective at preventing ED visits among younger adults than older ones. The median number needed to be vaccinated to prevent one ED visit ranged from 75 to 592 (median was 156) individuals.

Information from patients who had received either two or three mRNA vaccine doses was analyzed. None were immunocompromised. Data was from December 2021- February 2022, a period of Omicron BA.1 variant predominance.

“The reason why the number of patients needed to be vaccinated to prevent a COVID-19 related hospitalization is different from the number needed to prevent an ED (Emergency Department) visit is not fully understood, but it is likely because of how people seek health care. Many people, especially younger ones who lack health insurance or Medicare, are more likely to use the ED for primary health care. On the other hand, older people usually go to their regular doctor instead of going to the ED,” Dr. Grannis observed.

The paper is published in The Lancet Regional Health—Americas.

More information:
Katherine Adams et al, Number needed to vaccinate with a COVID-19 booster to prevent a COVID-19-associated hospitalization during SARS-CoV-2 Omicron BA.1 variant predominance, December 2021–February 2022, VISION Network: a retrospective cohort study, The Lancet Regional Health—Americas (2023). DOI: 10.1016/j.lana.2023.100530