Pfizer vaccine effectiveness diminishes against infection, not severe disease
The effectiveness of the Pfizer-BioNTech vaccine against any SARS-CoV-2 infection was about 20% among patients in Qatar 5 to 7 months after administration of the second dose, a study in The New England Journal of Medicine showed.
However, vaccine effectiveness against COVID-19-related hospitalization or death “persisted with hardly any waning for 6 months after the second dose,” researchers wrote.
In a second study also published in The New England Journal of Medicine, data from vaccine recipients in Israel revealed substantial declines in neutralizing antibody titers, particularly among certain populations, 6 months after receipt of the second Pfizer-BioNTech dose.
Vaccine efficacy rates
In the first study, researchers conducted a matched test-negative, case-control trial to analyze vaccine efficacy in 907,763 Qatar residents of various national origins (median age, 31 years; approximately 69% male) who received two Pfizer-BioNTech vaccine doses.
“We were unable to use a cohort study design owing to depletion of the unvaccinated cohorts by the high vaccine coverage,” Hiam Chemaitelly, MSc, a senior epidemiologist supervisor at Weill Cornell Medical College in Qatar, and colleagues wrote.
The study participants were matched based on several factors, including sex, 10-year age group and nationality. Most participants (97.4%) received both Pfizer-BioNTech COVID-19 vaccine doses within 30 days, according to the researchers.
The results indicated that vaccine effectiveness against any SARS-CoV-2 infection was 36.8% (95% CI, 33.2-40.2) in the third week after the first dose, peaking at 77.5% (95% CI, 76.4-78.6) in the first month following the second dose. Effectiveness gradually waned thereafter, then dropped after the fourth month to reach about 20% in the fifth through seventh months, according to the researchers.
“These findings suggest that a large proportion of the vaccinated population could lose its protection against infection in the coming months, perhaps increasing the potential for new epidemic waves,” Chemaitelly and colleagues wrote.
The findings conflict with other data published earlier this week in The Lancet, which showed that the effectiveness of the Pfizer-BioNTech vaccine declined to 47% (95% CI, 43–51) 5 months after administration of the second dose among a larger cohort of Americans.
Regarding severe, critical and fatal cases of COVID-19, Chemaitelly and colleagues reported that vaccine effectiveness increased rapidly to 66.1% (95% CI, 56.8-73.5) by the third week after the first dose, reaching 96% or higher in the first 2 months after the second dose. The “effectiveness persisted at approximately this level for 6 months,” according to the researchers.
Also, the effectiveness data remained unchanged after adjustment for COVID-19 variants, and there were no data to suggest an “appreciable waning” against hospitalization and death in the 6 months after administration of the second dose, the researchers wrote.
Neutralizing antibody titers
In the second study, researchers in Israel performed a longitudinal prospective trial among 3,808 vaccinated health care workers who were administered the Pfizer-BioNTech vaccine. Each month for 6 months, the health care workers were tested to ascertain anti-spike IgG and neutralizing antibody levels.
Einav G. Levin, MD, of the Sheba Medical Center in Tel Aviv, and colleagues wrote that the level of IgG antibodies dropped at a “consistent rate, whereas the neutralizing antibody level decreased rapidly for the first 3 months with a relatively slow decrease thereafter.”
Although there was a correlation between IgG antibody levels and neutralizing antibody titers, “the regression relationship between the IgG and neutralizing antibody levels depended on the time since receipt of the second vaccine dose,” the researchers continued.
After 6 months, neutralizing antibody titers were substantially lower among male health care workers compared with female health care workers (ratio of means = 0.64; 95% CI, 0.55-0.75), as well as health care workers aged 65 years and older compared with those aged between 18 to 45 years (ratio of means = 0.58; 95% CI, 0.48-0.7) and health care workers with immunosuppression compared with those who were not immunosuppressed (ratio of means = 0.3; 95% CI, 0.2-0.46).
“Published work about many vaccines, such as those against measles, mumps, and rubella, has shown a small decrease each year of 5 to 10% in the neutralizing antibody levels,” Levin and colleagues wrote. “We found that a significant and rapid decrease in humoral response to the [Pfizer-BioNTech] vaccine was observed within months after vaccination.”
As the pandemic continues, Levin and colleagues noted that “strategies to prolong host immunity need to be evaluated in order to protect the population against SARS-CoV-2 and its variants.”