evidence: Written by John Hickner, MD, MS, on January 26, 2021. (Source: Allergic reactions including anaphylaxis after receipt of the first dose of Pfizer-BioNTech COVID-19 Vaccine — United States, December 14–23, 2020. MMWR Morb Mortal Wkly Rep. 2021;70(2):46-51.) [...] Successful Chinese Phase 2 Trial of a COVID-19 Vaccine. These researchers identified 508 healthy volunteers and randomized them to low-dose vaccine (5 x 1010 viral particles), high-dose vaccine (1 x 1011 viral particles), or placebo vaccine in a 2:1:1 ratio (this was an adenovirus type-5 vectored vaccine). Allocation was concealed, and patients and outcome assessors were masked. The mean was 40 years of age, with a range of 18 to 83 years, but only 13% of recipients were 55 years or older. Seroconversion of the binding antibody was high at 96% to 97% in the two vaccine groups at 28 days. Seroconversion of the neutralizing antibodies to SARS-CoV-2 occurred in 59% receiving the high-dose vaccine and 49% receiving the low-dose vaccine at 28 days. Note that binding antibodies bind to the viral antigen but do not prevent infection of the host cell, whereas neutralizing antibodies prevent infection. Evidence of humoral (T-cell mediated) immunity was detected in 90% of patients receiving the high-dose vaccine and 88% with the low-dose vaccine at 28 days. Patients with high preexisting titers to adenovirus type 5 and older patients had less vigorous immune responses and would likely require a second immunization.