A review of 33 published and unpublished outbreak reports has quantified something health departments have long treated as a footnote: vaccinated people who catch measles can sometimes pass it on. Across the pooled reports, 70 of 890 vaccinated patients, or 7.9 percent, transmitted the virus to someone else, producing 237 secondary cases in total.
The analysis, led by researchers at Public Health Ontario and published in the journal Expert Review of Vaccines, is not an argument against vaccination. More than 92 percent of the vaccinated patients in the pooled data infected no one at all, and two doses of the measles, mumps, and rubella vaccine remain about 97 percent effective at preventing infection in the first place. The finding is about outbreak investigation, not about whether the shot works.
