Prepared by the North American health company Kaiser Permanente Southern California (KPSC) and the pharmaceutical company Pfizer, the study says that the two doses of the vaccine prevent hospitalizations in 90% of cases of infection in the 24 weeks following the inoculation and that this then drops to 47% after that interval.
The investigators found that the drop in efficacy over time does not mean the virus “evades” the protection offered by the vaccine, but they do warn about the delta variant became dominant during the study, recommending analyzes to measure the rate of decline of this vaccine.
During the investigation, researchers examined 3,436,957 electronic health records from Kaiser Permanente Southern California (KPSC) between December 4, 2020 and August 8, 2021.
During this period, 5.4% of the individuals were infected by the new coronavirus and, within this group, 6.6% were hospitalized, after an interval, on average, of three to four months between taking the first and taking the second dose.
Analysis of the complete genomic sequence and viral information from 8,911 positive PCR tests revealed that the delta variant accounted for 28% of the total proportion of positive sequences.
The proportion of positive cases attributed to this variant increased from 0.6% detected in April 2021 to almost 87% last July, which confirmed the predominance of delta in the United States.
"Our study confirms that vaccines are a fundamental tool to control the pandemic and remain highly effective in preventing serious illness and hospitalizations for delta variant or other variants of concern," said lead author of the study, Sara Tartof, from KPSC.
The specialist also acknowledged that protection against the infection falls in the months after the second dose of the vaccine, so she asks the competent authorities to demand more studies to determine whether it is necessary to boost the vaccine in "all age groups".