The effectiveness of the Pfizer-BioNTech COVID-19 vaccine dropped from 96 percent to 84 percent over six months, a new study funded by the companies showed.
The study, published on Wednesday, showed that while the efficacy level of the Pfizer vaccine reached a peak of 96.2 percent within the first two months after the second dose, it “declined gradually” in the coming four months.
Its efficacy decreased by about six percent every two months, according to the study which looked at the immune response of over 44,000 participants.
If it continues to fall at its current rate, its efficacy could decrease to 50 percent within 18 months – which would make booster shots necessary.
The new study did not take into account the effect of the more infectious strain of the virus – the Delta variant – on the vaccine’s efficacy.
However, the Pfizer vaccine still provided adequate protection against the coronavirus despite the drop in efficacy and is overall 91.1 percent effective against COVID-19, according to the researchers.
Only 81 people who had received the vaccine during the study were later infected compared to 873 people who had received a placebo.
According to health experts, vaccinated people are not immune to the disease, but getting inoculated reduces the severity of the illness.
‘Breakthrough cases’ – where vaccinated individuals are infected with COVID-19 – often report mild or no symptoms.
The Pfizer vaccine showed a 97 percent efficacy rate against severe cases of the virus – making it less likely that those who received two doses would be hospitalized or suffer from serious symptoms.
The researchers will continue to analyze participants’ immune responses to the disease for up to two years to determine “whether a booster is likely to be beneficial after a longer interval.”
“Ongoing follow-up is needed to understand persistence of the vaccine effect over time, the need for booster dosing, and timing of such a dose,” the researchers said.
Earlier this month, Pfizer announced that it has plans to apply for approval for a third booster shot. At the time the Food and Drug Association (FDA) had said booster shots were not necessary “at this time.”