A study by the Pfizer laboratory, in partnership with the American health network Kaiser Permanente, published on 5 October in the journal The Lancet, confirms that the two doses of Pfizer/BioNTech vaccine are effective against the risk of infection with COVID-19 for a period of six months. This reopens the debate on a future third dose of the vaccine.
How long is the Pfizer vaccine effective?
Pfizer researchers looked at medical data from 3.4 million people in Southern California between December 4, 2020 and August 8, 2021. The results validate previous estimates of how long the Pfizer vaccine will be effective.
While the efficacy against the risk of infection is 88% in the month following the 2nd injection of the vaccine, it is almost halved 6 months later. In fact, it is only 47% according to the laboratory's study.
The lead author of the paper, Sara Tartof, says:
Our study confirms that vaccines are a central tool for controlling the epidemic and remain effective in preventing severe disease and hospitalization, including against Delta or other variants of concern.
Biases in the Pfizer survey
Pfizer's vice-president Luis Jodar says:
A variant-specific analysis clearly shows that the vaccine is effective against all kinds of variants. People infected with COVID-19 who had received two doses of vaccine were most often infected because of a loss of vaccine efficacy over time and not because one or another variant escapes the protection of the vaccine.
However, the scientists point out that their investigation was biased. In addition to the fact that this study was carried out by researchers from the Pfizer laboratory, which markets the vaccine, they acknowledge that they did not have any data from the patients analysed concerning compliance with the wearing of masks, social interactions or even the profession of the populations studied (and therefore the impact of human contact), which may be factors in contamination by the virus.