Study Finds COVID-19 Vaccine Efficacy Declines After 6 Months Without Boosters


COVID-19 vaccines are effective in preventing severe infection resulting in hospitalization, but protection wanes after 6 months.

A new study from clinicians and scientists at the Providence Research Network found that the overall efficacy of vaccines in preventing severe infection resulting in hospitalization from COVID-19 is positive, but it also shows a substantial decline in protection after 6 months.

The study analyzed data from nearly 50,000 hospital admissions between April and November 2021, which highlighted vaccines were 94% effective at preventing hospitalization 50 to 100 days after receiving the shot but fell to 80.4% 200 to 250 days later, with even more rapid declines after 250 days.

Additionally, the study identified the factors linked with reducing vaccine efficacy. The key risk factors that qualified for a severe breakthrough infection included advance age, comorbidities, transplants, chronic kidney disease, hypertension, or heart failure, the amount of time that had elapsed since being vaccinated, and the type of vaccine an individual received.

Although the Moderna vaccine offered the best overall protection over the course of the study, the Pfizer-BioNTech vaccine offered initial protection equivalent to the Moderna vaccine but declined faster over time. Those who received the Janssen vaccine also had higher odds of experiencing a severe breakthrough infection compared to Moderna, according to the study.

“This data helps us understand differences in waning protection by vaccine type and identify the key risk factors for severe breakthrough infections to help inform the targeting of potential vaccine booster programs,” said Amy Compton-Phillips, MD, Providence chief clinical officer, in a press release. “Unlike most other studies, our data stretched beyond 6 months, where we found evidence of rapidly waning protection, especially for patients 80 or older. We were also able to identify important differences by vaccine type and patient characteristics that should help inform potential booster programs.”

The recent data are intended to support the importance of vaccinations for protection against hospitalization and the need to boost protection after 200 days, specifically for patients who are 80 years of age and older or who have certain medical conditions that increase their risk of severe infection.

“Additional protection may be warranted for everyone, but especially for these populations,” said Ari Robiscek, MD, Providence chief medical analytics officer, in a press release. “In addition to promoting general vaccine uptake, clinicians and policy makers should consider prioritizing booster shots toward those most at risk for severe COVID-19.”

Another study shows that vaccinated patients with COVID-19 who were admitted to the hospital during the Omicron variant surge were found to have less severe illness than unvaccinated patients and were also less likely to be admitted to intensive care.

An evaluation of clinical information from electronic health records showed that a greater portion of the patients hospitalized when Omicron was dominant were vaccinated compared to patients hospitalized during summer 2021, when the Delta variant was dominant. This likely reflected the higher percentage of the populations who were vaccinated during the Omicron wave, according to the study.

Getting vaccinated against COVID-19, including a booster dose for those who are fully vaccinated, remains critical for mitigating the risk of severe illness associated with SARS-CoV-2 infection, according to the researchers.


Providence study: COVID vaccine effectiveness declines after 6 months without boosters. EurekAlert! February 25, 2022. Accessed February 28, 2022.

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