In adolescents immunized with two doses, the vaccine's effectiveness in preventing symptomatic Covid-19 already begins to decline after 27 days, while protection against severe forms of the disease remains high for over 3 months.
This is the data that emerges from a study carried out in Brazil and Scotland and published in Lancet Infectious Disease.
The research considered over 600,000 tests carried out on 3.5 million children between 12 and 17 years old vaccinated with two doses of Pfizer vaccine (BNT162b2).
Although differences were found between the two countries, the trends identified by the researchers were overlapping.
The efficacy of the vaccine peaked between 14 and 27 days after administration of the second dose and was lower during the period when the Omicron variant was in circulation than during the Delta-dominated period.
In particular, in the Omicron phase the peak efficacy against symptomatic disease was 64.7% in Brazil and 82.6% in Scotland.
In the Delta period, the efficacy was 80.7% and 92.8% respectively.
In both countries the effectiveness started to decline after the twenty-seventh day, falling, after 98 days, to 5.9% in Brazil and 50.6% in Scotland.
The efficacy against severe forms of Covid, on the other hand, remains high for a long time, remaining above 82% after 98 days from the second dose.
"In summary, our results indicate that in adolescents the protection against symptomatic infection with the Omicron variant decreases rapidly over time after two doses of BNT162b2 and, therefore, two doses are not sufficient to support protection against symptomatic disease." the researchers write.
"However, protection against severe disease likely remains high at 98 days or more after the second dose. Our findings support the importance of maximizing vaccination coverage and considering booster doses for adolescents," they conclude.
