COVID-19 vaccine boosters bolster protection against hospitalization but the effect wanes over time, according to a study published Feb. 11.
Researchers with the Centers for Disease Control and Prevention (CDC) and other institutions found the booster effectiveness against emergency department or urgent care (ED/UC) visits held at 87 percent during the first two months after a third dose of the Pfizer or Moderna vaccine, but dropped to 66 percent between four and five months after receipt of the booster.
Protection against hospitalization also decreased over time, from 91 percent in the first two months after the booster to 78 percent four or more months after the third dose.
The Moderna and Pfizer vaccines are both built on messenger RNA (mRNA) technology.
Both were authorized by U.S. regulators in December 2020 and both are administered in two-dose primary regimens spaced several weeks apart.
But the vaccines proved less effective against both infection and hospitalization after the Delta virus variant became dominant in the United States, and have been even less effective against the Omicron variant, which displaced Delta in December 2021. That's led U.S. health authorities, including the CDC, to recommend three doses of the Pfizer and Moderna jabs, and some officials have hinted that a second booster will be recommended in the future.
Researchers conducting the new study analyzed COVID-19 patients who sought medical care between Aug. 26, 2021, and Jan. 22, 2022. They pulled data from the VISION Network, a surveillance system managed by the CDC.
The increases in protection for boosters, even with the waning, show the importance of being "up to date with recommended COVID-19 vaccinations," the researchers said.
Before being boosted, protection from vaccination against ED/UC visits due to COVID-19 was 69 percent within 2 months of the second shot, but dropped to just 37 percent at 5-plus months after the second shot, researchers found.
Additionally, the primary regimens of the Moderna or Pfizer shots were 71 percent effective within 2 months of the second dose and dropped to just 54 percent after 5-plus months.
Illustrating the waning effectiveness of the vaccines, researchers also found that of 241,204 eligible ED/UC visits, 54 percent were among the vaccinated, and among the 93,408 eligible hospitalizations, 57 percent were among the vaccinated.
Limitations of the study include not examining the vaccine effectiveness against COVID-19 infections that did not lead people to seek medical care.