Closeup of clinician's hands getting vaccine dose ready

The protection derived from a booster shot of the Moderna or Pfizer-BioNTech vaccines wanes substantially after four months, although some efficacy is retained, federal researchers have found.

In a multistate study, mRNA vaccination was 87% effective against emergency department and urgent care visits and 91% against hospitalizations within two months after a third dose. Efficacy then declined to 66% efficacy against ED/UC visits and 78% for hospitalizations at month four, according to a new study released Friday by the Centers for Disease Control and Prevention.

The results may support recommending additional doses of mRNA vaccines to maintain or boost protection against severe COVID-19 illness, the authors said. 

In any case, all eligible persons should remain up to date with recommended COVID-19 vaccinations to best protect against COVID-19–associated adverse outcomes, they concluded.

Three shots better for seniors

Another new study has a similar takeaway. Health insurer Kaiser Permanente has shown that three doses of Pfizer’s vaccine are better than two — especially for older adults.

Investigators examined health records from 3.1 million beneficiaries in Southern California from Dec. 14, 2020, to Dec. 5, 2021. They found that two-dose vaccination effectiveness against infection with SARS-CoV-2 declined from 85% during the first month after vaccination to 49% up to eight months afterward. 

In addition, two doses provided long-term protection against hospitalization remained notably high, at 90% overall, but not in patients aged 75 years and older, or for those with compromised immune systems, the researchers reported. For these patients, efficacy fell to approximately 75%.

In contrast, three-dose vaccination remained 88% effective against infection and 97% against hospitalization within the first three months after patients received the shots.

“[T]he public health impact of a third dose to prevent severe disease is substantial,” said said Sara Y. Tartof, Ph.D., an epidemiologist. “Importantly, all studies that have evaluated the vaccine effectiveness of a third dose — including ours — have shown a meaningful improvement in vaccine effectiveness against a broad range of SARS-CoV-2 outcomes.” 

The study was published in the Lancet Regional Health – Americas.