Study Interpretation

BCG-Denmark vaccination had a beneficial effect on herpes labialis, particularly in males with frequent recurrences, but may increase the risk of a first cold sore.

Study Results

Results: Between March 30, 2020, and February 18, 2021, 84 individuals with frequent recurrent cold sores were randomly assigned to BCG (n = 38) or control (n = 46). The average time to first cold-sore recurrence was 1.55 months longer in the BCG group (95% CI 0.27–2.82, p = 0.02) than the control group (hazard ratio 0.54, 95% CI 0.32–0.91; intention-to-treat). The beneficial effect of BCG was more significant in the as-treated population (difference 1.91 months, 95% CI 0.69–3.12, p = 0.003; hazard ratio 0.45, 95% CI 0.26–0.76). In prespecified subgroup analyses, only sex modified the treatment effect (interaction p = 0.007), with benefits restricted to males. Over 12 months, a more significant proportion of participants in the BCG group compared with the control group reported a decrease in duration (61% vs. 21%), severity (74% vs. 21%), frequency (55% vs 21%), and impact on quality of life (42% vs 15%) of cold sore recurrences. In participants who had ever had a cold sore, there was also a decrease in self-reported burden of recurrences in the BCG group. In participants who had never had a cold sore, there was an increased risk of a first episode in the BCG group (risk difference 1.4%; 95% CI 0.3–2.6%, p = 0.02). There were no safety concerns.