The Pfizer-BioNTech COVID-19 vaccine contains a nucleoside-modified messenger RNA ( modRNA ) encoding the viral spike glycoprotein ( S ) of SARS-CoV-2.
The vaccine has been shown to elicit increased local and systemic adverse reactions as compared to those in the placebo arm, usually lasting a few days.
The most common solicited adverse reactions were injection site reactions ( 84.1% ), fatigue ( 62.9% ), headache ( 55.1% ), muscle pain ( 38.3% ), chills ( 31.9% ), joint pain ( 23.6% ), fever ( 14.2% ).
Adverse reactions characterized as reactogenicity were generally mild to moderate.
The number of subjects reporting hypersensitivity-related adverse events was numerically higher in the vaccine group compared with the placebo group ( 137 [ 0.63% ] vs 111 [ 0.51% ] ).
Severe adverse reactions occurred in 0.0-4.6% of participants, were more frequent after Dose 2 than after Dose 1 and were generally less frequent in older adults ( more than 55 years of age ) ( less than 2.8% ) as compared to younger participants ( less than or equal to 4.6% ).
Among reported unsolicited adverse events, lymphadenopathy occurred much more frequently in the vaccine group than the placebo group and is plausibly related to vaccination.
Serious adverse events, while uncommon ( less than 1.0% ), represented medical events that occur in the general population at similar frequency as observed in the study.
Three serious adverse events in the BNT162b2 group were considered related by the investigator, but not the Sponsor, as related to study vaccination: shoulder injury ( n=1 ), ventricular arrhythmia in a participant with known cardiac conditions ( n=1 ), and lymphadenopathy temporally related following vaccination ( n=1 ).
FDA has considered two of the events as possibly related to vaccine: the shoulder injury possibly due to vaccine administration or the vaccine itself and lymphadenopathy. Lymphadenopathy was temporally associated and biologically plausible.
No specific safety concerns were identified in subgroup analyses by age, race, ethnicity, medical comorbidities, or prior SARS-CoV-2 infection.
Although participants 16 to 17 years of age were enrolled in the phase 3 trial, safety data for this age group is limited.
However, available data are consistent with the safety profile in the adult population, and it is biologically reasonable to extrapolate the greater safety experience in adults, in particular younger adults, to the oldest pediatric age group of 16 to 17 years.
Unknown risks / Data gaps
Safety in certain subpopulations - There are currently insufficient data to make conclusions about the safety of the vaccine in subpopulations such as children less than 16 years of age, pregnant and lactating individuals, and immunocompromised individuals.
Adverse reactions that are very uncommon or that require longer follow-up to be detected - Following authorization of the vaccine, use in large numbers of individuals may reveal additional, potentially less frequent and/or more serious adverse events not detected in the trial safety population of nearly 44,000 participants over the period of follow up at this time. Active and passive safety surveillance will continue during the post authorization period to detect new safety signals.
A numerically greater number of appendicitis cases occurred in the vaccine group but occurred no more frequently than expected in the given age groups and do not raise a clear concern at this time for a causal relationship to study vaccination.
Although the safety database revealed an imbalance of cases of Bell’s palsy ( 4 in the vaccine group and none in the placebo group ), causal relationship is less certain because the number of cases was small and not more frequent than expected in the general population. Further signal detection efforts for these adverse events will be informative with more widespread use of the vaccine.
Available data do not indicate a risk of vaccine-enhanced disease, and conversely suggest effectiveness against severe disease within the available follow-up period.
However, risk of vaccine-enhanced disease over time, potentially associated with waning immunity, remains unknown and needs to be evaluated further in ongoing clinical trials and in observational studies that could be conducted following authorization and/or licensure. ( Xagena )
Source: FDA, 2020