Full vaccination with the two-dose Pfizer COVID-19 vaccine has been 90% effective in protecting against hospitalization for at least six months, a new study shows.

That includes the time during which the highly contagious Delta variant has been circulating in the United States.

For the study, researchers analyzed Kaiser Permanente Southern California data on more than 3.4 million people who received both doses of the Pfizer vaccine. Their average time since being fully vaccinated was three to four months.

During the study period — from Dec. 4, 2020 to Aug. 8, 2021 — 5.4% of those people were infected with the coronavirus. Of those, 6.6% were hospitalized.

Overall, vaccine effectiveness against all SARS-CoV-2 infections fell from 88% within one month after receiving two vaccine doses to 47% after six months, but effectiveness against hospitalizations remained at 90%, the findings showed.

Vaccine effectiveness against all Delta variant infections fell from 93% after one month to 53% after four months, and effectiveness against other variants fell from 97% at one month to 67% after four months. But effectiveness against Delta-related hospitalizations remained at 93% over the study period.

The findings, published Oct. 4 in The Lancet medical journal, are consistent with preliminary reports from the U.S. Centers for Disease Control and Prevention and the Israel Ministry of Health showing declines in the Pfizer vaccine's protection against infection after about six months.

"Our study confirms that vaccines are a critical tool for controlling the pandemic and remain highly effective in preventing severe disease and hospitalization, including from the Delta and other variants of concern. Protection against infection does decline in the months following a second dose," study lead author Dr. Sara Tartof, from the department of research & evaluation at Kaiser Permanente Southern California, said in a journal news release.

"While this study provides evidence that immunity wanes for all age groups that received the vaccine, the CDC Advisory Committee on Immunization Practices has called for additional research to determine if booster shots should be made available to all age groups eligible for this vaccine," she noted.

"In line with the recent [U.S. Food and Drug Administration] and CDC recommendations, considerations for booster shots should take global COVID-19 vaccine supply into account as people in many countries around the world have not yet received a primary vaccination series," Tartof added.