The Centers for Disease Control and Prevention released data on Friday, indicating that the level of protection against Covid hospitalizations provided by the Pfizer-BioNTech vaccine has declined significantly in the four months after full inoculation.
The data was released hours before a Food and Drug Administration scientific advisory committee overwhelmingly recommended approving a booster injection of the Pfizer-BioNTech vaccine for people 16 and older. However, the debate over booster shots will continue as time goes on and more data is collected.
The new study found that from two weeks after recipients received their second dose — a point at which they are normally considered fully vaccinated — to four months later, the Pfizer vaccine was 91 percent effective at preventing hospitalization. However, after 120 days, the effectiveness dropped to 77 percent.
The Moderna vaccine did not show a comparable decrease in protection over the same period: It was 92 percent effective against hospitalizations four months after the recipients were vaccinated, a level nearly identical to the 93 percent effectiveness before.
The study said not enough participants had received the one-time Johnson & Johnson vaccine to compare performance. Overall, however, the Johnson & Johnson admission was 71 percent effective in preventing hospitalizations.
The CDC study released Friday supported some others who suggested that the Pfizer vaccine may offer less protection against hospitalization over time. But the available data is far from unanimous.
Other studies have shown that the effectiveness of Pfizer against hospitalization has remained above 90 percent, despite the spread of the Delta variant and the longer time since people received their second injection. Pfizer has said data from Israel points to declining effectiveness against serious illness, although it seems that Israel and the United States define “serious illness” differently.
The latest CDC study was based on an analysis of about 3,700 adults who were hospitalized in the United States from March to August.
People with compromised immune systems, who typically do not respond well to vaccines, were excluded from the study. Nevertheless, the vaccinated patients were mostly older people — the Pfizer cohort had a median age of 68 — and it was unclear whether the vaccine’s effectiveness had changed much in younger age groups. Previous studies have shown lower levels of protection in the elderly.
The study authors said the gap in the performance of the Moderna and Pfizer vaccines could stem from a higher dose of mRNA in the Moderna injections or the four-week gap between doses of the Moderna vaccine. (Pfizer vaccines were given three weeks apart.) It’s also possible, they said, that other, undetected differences in the study participants who received either injection could also have influenced the results.