Study: 2 Doses of Pfizer COVID-19 Vaccine Provide Protection for 4 Months in Adolescents

Effectiveness is estimated at approximately 91% against any SARS-CoV-2 infection and approximately 85% effective against asymptomatic infection, analysis results show.

Two doses of the BNT162b2 (Pfizer, BioNTech) COVID-19 vaccine provided protection for at least 4 months after immunization against symptomatic and asymptomatic SARS-CoV-2 infections in US adolescents, results of a study published in JAMA Network Open showed.

In the study, investigators tested 6901 adolescents for SARS-CoV-2, with 186 individuals and 356 matched control individuals included.

The median age of individuals in the study was 14, approximately 48% identified as female, 15% were Black, 15% were Hispanic, and 55% were white.

Additionally, approximately 25% of participants were fully immunized, with the mean time between immunization and a SARS-CoV-2 test 62 days.

Within 4 months of receiving both doses, the vaccine’s effectiveness was estimated to be approximately 91% against any infection and approximately 85% effective against asymptomatic infection. The effectiveness after a single dose was estimated at approximately 74%.

After a second dose, the estimated vaccine effectiveness against any infection peaked between week 9 and 12, with the lowest rate being between week 13 and 17.

The adjusted estimates of vaccine effectiveness (aVE) after 2 doses were approximately 90%, and the 2 doses were slightly less effective against asymptomatic infection.

Investigators also analyzed the cases infected with the Delta variant and found that after 2 doses, the vaccine effectiveness was approximately 94%.

The study included immunization histories, relevant clinical data, and RT-PCR test results that were obtained from the Yale New Haven Health System during the study time frame between June 1, 2021, and August 15, 2021, when the Delta variant was extremely prevalent in Connecticut.

The case individuals were adolescents who had a positive test result and an associated medical encounter, investigators said.

The individuals in the control group were all those who had negative test results and were matched to a case participant based on age, county of residence, and date of testing.

Individuals in both groups who received an influenza vaccine during the preceding season were the same, investigators said.

Individuals were considered fully immunized if they received 2 doses at least 14 days before focal time.

The investigators reported 9 hospitalizations, with 7 case individuals and 2 control individuals. Just 1 individual was admitted to an intensive care unit. Additionally, none of the 10 individuals with breakthrough infections were hospitalized.

The primary outcome was measured by the SARS-CoV-2 infection via RT-polymerase chain reaction tests. Meanwhile, vaccine effectiveness was determined by using matched odds ratios from conditional logistic regression models.

The secondary outcomes included the estimated vaccine effectiveness by clinical symptoms, number of vaccine doses received, and time from immunization.

Investigators acknowledged the limitations of the study, primarily the potential for residual confounding, which is a limitation found in all observational studies.

However, the investigators said that they focused on analytics and the study design to minimize the risk as much as possible.

Reference

Oliveira CR, Niccolai LM, Sheikha H, Elmansy L, Kalinich CC, Grubaugh ND, Shapiro ED; Assessment of clinical effectiveness of BNT162b2 COVID-19 vaccine in US adolescents. JAMA Netw Open. 2022;5(3):e220935. doi:10.1001/jamanetworkopen.2022.0935