Background
Side effects to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccines are a key concern contributing to vaccine hesitancy, but more individuals may be encouraged if SARS-CoV-2 vaccines were known to lead to a stronger immune response.

Methods
Included were adult participants from the Danish National Cohort Study of Effectiveness and Safety of SARS-CoV-2 Vaccines (ENFORCE) who completed a questionnaire to assess systemic reactions following SARS-CoV-2 vaccination (BTN162b2, mRNA-1273, ChAdOx1) and had SARS-CoV-2 spike immunoglobulin G (IgG) levels measured at baseline and post–vaccine. A symptom score was developed to measure severity of systemic adverse reactions (+1 for each moderate, +2 for each severe). Post–vaccination SARS-CoV-2 spike IgG levels were compared between participants with different scores using multivariable linear regression.

Results
A total of 6528 participants were included (56.3% females; median age [interquartile range], 64 [54–75] years). After the first vaccination, no association was found between symptom score and post–vaccine dose spike IgG level (P = .575). Following the second vaccination, significantly higher spike IgG levels were observed according to higher symptom scores (P < .001); adjusted geometric mean ratios were 1.16 (95% CI, 1.04–1.30), 1.24 (95% CI, 1.09–1.41), 1.25 (95% CI, 1.06–1.46), and 1.21 (95% CI, 1.08–1.35), for scores of 2, 3, 4, and ≥5, respectively, compared with a score of 0. After adjustment for pre–vaccine dose spike IgG, this association was attenuated.

Conclusions
An association was found between more severe adverse reactions and stronger antibody response after the second vaccination but not the first, likely attributed to higher levels of preexisting immunity gained from response to first vaccination. Regardless of side effects, most people experienced an effective immune response following vaccination.