The effectiveness of a COVID-19 vaccine booster against hospitalization drops sharply within months, according to a new study.
The effectiveness of a two-dose primary series of the Moderna or Pfizer vaccines after 14 months was just 19 percent, according to the new paper. That protection jumped to 89 percent after a booster dose but dropped to 66 percent between four and six months and to 31 percent after eight months.
The protection was the worst for young adults, declining to just 33 percent after four months, researchers found. The protection was more sustained for people aged 65 or older but still declined to less than 50 percent after eight months.
The effectiveness estimates were against COVID-19-associated hospital admissions, or hospitalizations where the person who was admitted tested positive for COVID-19.
Protection against COVID-19-associated emergency department and urgent care visits waned even faster, starting at 63 percent and dropping to 44 percent after several months. It bottomed out at 12 percent between 14 and 16 months. A third dose increased protection to 83 percent, but that dropped to lower than 50 percent between four and six months and just 17 percent after eight months.
Researchers with the Centers for Disease Control and Prevention (CDC) and other institutions performed the study, analyzing data from the CDC-funded VISION network, made up of health care institutions that provide and help analyze the data.
Pfizer and Moderna didn't respond to requests for comment.
The data were from hospitals, emergency departments, and urgent care clinics in 10 states from Jan. 17, 2021, to July 12, 2022. The test-negative case-control study included people admitted to hospitals, emergency departments, or urgent care who tested positive or negative for COVID-19. Researchers excluded a number of groups, including anybody who tested positive for COVID-19 but had no COVID-19 symptoms.
The estimates mentioned above were from December 2021, after the Omicron coronavirus variant became dominant in the United States.
The COVID-19 vaccines were much more effective against hospitalization from earlier variants, according to the new study and others like it.
"During the Omicron period, vaccine effectiveness waned within six months of the third dose by about 20 percentage points among those without immunocompromising conditions and by more than 40 percentage points among those with immunocompromising conditions. This amount of waning is enough to be relevant for clinical and policy considerations about the need for boosters or other protective measures," researchers said.
U.S. adults aged 50 and older and younger people who have certain conditions are advised by U.S. authorities to get a second booster dose, or fourth vaccine shot.
Early estimates for the effectiveness of a second booster indicated that it improved protection. For people aged 65 and older, the protection was 76 percent within two months of the second booster and 70 percent from two to four months.