Seventy-three of the 194 measles cases recorded in California in 2015—the outbreak that sparked the kick-back against the anti-vax movement—were merely reactions to the MMR (measles-mumps-rubella) vaccine.
Although the data has never been published, researchers from the US's Centers for Disease Control and Prevention (CDC) have confirmed that the analysis is correct.
It had been assumed that around 5 per cent of vaccinated children will display measles-like symptoms, such as rash or fever, but the new data suggests this reaction may be far more common.
If that's the case, more than a hundred of the 350 cases reported so far this year in the US could also be vaccine reactions.
Unvaccinated children have been blamed for the rise in measles cases, and health authorities fear the cherished herd immunity effect—where 95 per cent of a population is vaccinated—isn't being achieved because of the anti-vaccine stories on social media. Several states in the US are considering a restriction on exemptions against vaccination.
CDC researchers, working alongside scientists from the Public Health Agency in Canada, have been testing two new systems that can accurately assess whether a case of measles is caused by the virus or is a reaction to the vaccine. "It is very important to identify vaccine reactions to avoid unnecessary isolation of the patient," the researchers say.
Measles outbreaks in Alberta and British Columbia in Canada were two other instances when an accurate monitoring system could have quickly assessed whether they were cases of 'real' measles or not.