BNT162b1 is a lipid nanoparticle–encapsulated, nucleoside-modified mRNA vaccine candidate from Pfizer/BioNTech against SARS-CoV-2, administered by intramuscular injection. Its mRNA encodes the spike glycoprotein's receptor-binding domain (RBD), which is trimerized via an added phage T4 fibritin–derived trimerization domain to raise immunogenicity; incorporating 1-methylpseudouridine increases translation efficiency while still engaging the innate immune response. Because the mRNA does not integrate into the host genome and is only transiently translated to produce the spike antigen that triggers immunity, the platform is considered relatively safe. In the phase 1/2 trials, healthy adults aged 18–55 received the vaccine at 10, 30, or 100 µg or placebo; IgG binding and spike neutralization were dose-dependent. About 7 days after vaccination, both vaccine and placebo recipients reported mild-to-moderate fatigue and headache—more frequently among vaccine recipients—with no fatal events observed. Safety follow-up continues for up to ~6 months and immunogenicity for up to 2 years, and adults over 65 have begun enrolling.