The Pfizer-BioNTech COVID-19 vaccine has been a cornerstone in the global fight against the pandemic, with extensive data collected on its safety and efficacy. Clinical trials and real-world studies have provided valuable insights into its performance, side effects, and effectiveness against different SARS-CoV-2 variants. Regulatory bodies worldwide have continuously reviewed this data, affirming the vaccine's benefits in preventing COVID-19 and its associated
morbidity and mortality.
Reported Side Effects and Safety Monitoring
Most side effects associated with the Pfizer-BioNTech COVID-19 vaccine are generally mild to moderate and typically resolve within a few days. These reactions are considered normal signs that the body is developing protection against the virus and are similar to those experienced with other adult vaccines. During clinical trials, common side effects affecting more than one in ten people included pain and swelling at the injection site, tiredness, headache, muscle aches, chills, joint pain, fever, or diarrhea. Fever was observed to be more common after the second dose.
Regulatory agencies, such as the European Medicines Agency (EMA), have continuously monitored the vaccine's safety. A safety report published by the EMA in September 2021, based on over 392 million doses administered in the European Union, concluded that the benefits of the vaccine in preventing COVID-19 continue to outweigh any risks. Rare side effects, potentially affecting up to one in 1,000 people, have included temporary one-sided facial drooping and allergic reactions like hives or facial swelling.
Concerns regarding myocarditis and pericarditis (inflammation of the heart muscle and the sac surrounding the heart, respectively) have been reported, particularly in young men aged 16-30. For instance, Israel's Ministry of Health announced a probable relationship between the second dose and myocarditis in this demographic in June 2021, noting 55 cases per 1 million vaccinated individuals between December 2020 and May 2021, with 95% classified as mild. Similar reports emerged in the U.S. from April 2021, with about 13 cases per 1 million young people, mostly male and over 16, after vaccination with either the Pfizer-BioNTech or Moderna vaccine. Most affected individuals recover quickly with adequate treatment and rest.
Vaccine Effectiveness Against Variants and Over Time
The effectiveness of the Pfizer-BioNTech vaccine has been rigorously studied against various SARS-CoV-2 variants. A test-negative case-control study published in August 2021 found that two doses of the BNT162b2 vaccine had 93.7% effectiveness against symptomatic disease caused by the Alpha (B.1.1.7) variant and 88.0% effectiveness against symptomatic disease caused by the Delta (B.1.617.2) variant. Effectiveness after one dose was lower, at 48.7% against Alpha and 30.7% against Delta.
However, studies have also indicated a decline in effectiveness over time. The U.S. Centers for Disease Control and Prevention (CDC) reported in August 2021 that effectiveness against infection decreased from 91% to 66% when the Delta variant became predominant in the U.S., potentially due to waning immunity. Preliminary data suggested that effectiveness against the Omicron variant starts to decline in about 10 weeks after either the initial two-dose regimen or a booster dose, while for other variants, the effectiveness of initial doses begins to wane around six months. A case-control study in Qatar observed that effectiveness against infection peaked at 78% in the first month after the second dose, declining to 20% at months 5 to 7. Nevertheless, effectiveness against severe disease, hospitalization, and death remained more robust, peaking at 96% in the second month and remaining stable through the sixth month before declining.
Booster doses have shown to restore protection. A Phase III trial in October 2021 demonstrated that a booster dose given approximately 11 months after the second dose restored protective effect to 96% efficacy against symptomatic Delta variant disease. Real-world data from South Africa in December 2021 indicated that two doses provided about 70% effectiveness against hospital admission and 33% against symptomatic disease from the Omicron variant, with protection against hospital admission maintained across all ages and groups with comorbidities. Preliminary data from Pfizer and BioNTech also suggested that a third dose would provide a similar level of neutralizing antibodies against Omicron as seen after two doses against other variants. A study in Finland (September 2022–January 2023) showed that a bivalent booster reduced the risk of severe COVID-19 outcomes among the elderly, with highest protection observed during the first two months post-vaccination.
