New Study Reveals Pfizer's COVID-19 Vaccine Effectiveness Drops By 47 Percent After Six Months

Illustration of Pfizer's COVID-19 vaccine. (Wikimedia Commons/US Secretary of Defense)

JAKARTA - The effectiveness of Pfizer's COVID-19 vaccine in preventing has been found to have decreased, according to data published on Monday, as the United States health agency considers whether a booster dose is needed.

In data published in the media journal 'Lancet', previously released last August for peer review, the vaccine's effectiveness in preventing infection fell to 47 percent from 88 percent six months after the second dose.

The analysis showed the vaccine's effectiveness in preventing hospitalization and death remained high, up to 90 percent for at least six months, including against the highly contagious Delta variant of the coronavirus.

The data suggest this decline was due to reduced efficacy, rather than the more contagious variant of COVID-19, the researchers said.

Researchers from Pfizer and Kaiser Permanente studied the electronic health records of the approximately 3.4 million people who were members of Kaiser Permanente Southern California between December 2020, when the vaccine was first available, and August 2021.

"Our variant-specific analysis clearly shows that the (Pfizer) vaccine is effective against all the variants of current concern, including the Delta variant," said Luis Jodar, senior vice president and chief medical officer at Pfizer vaccines.

A potential limitation of this study is the lack of data on adherence to mask use and occupational guidelines in the study population, which could affect the frequency of testing and the likelihood of exposure to the virus.

The vaccine's effectiveness against the Delta variant was 93 percent after the first month, decreasing to 53 percent after four months. Against other coronavirus variants, efficacy decreased to 67 percent from 97 percent.

"To us, it shows the Delta variant is not an escape variant that completely evades vaccine protection," said study leader Sara Tartof of the Kaiser Permanente Southern California Department of Research & Evaluation.

"If it is, we probably won't see high protection after vaccination, because vaccination won't work in that case. It will start low, and stay low."

Testing for variants was more likely to fail in individuals who received the COVID-19 vaccine, which could lead to an overestimation of the variance-specific effectiveness in this study, the authors warn.

Separately, the United States Food and Drug Administration (FDA) has authorized the use of booster doses of the Pfizer/BioNTech vaccine for older adults and some Americans who are at high risk of infection.

Meanwhile, scientists have asked for more data on whether boosters should be recommended for all.