Of the people who got sick and for whom the state could determine vaccination status, 57 people hadn't been vaccinated against measles and 20 people had had at least one shot of the vaccine.
Researchers analyzed the California outbreak data as well as information gleaned from news reports and the Internet to figure out how big a factor the lack of vaccination was. The short answer, as you might have guessed, is big.
"This preliminary analysis indicates that substandard vaccination compliance is likely to blame for the 2015 measles outbreak," she and her co-authors wrote in a research letter that was published online Monday by JAMA Pediatrics. By their calculations, vaccination rates among the people exposed to the infection might have been as low as 50 percent and probably not more than 86 percent.
The rapid spread of the measles in California and beyond has put a spotlight on vaccination rates.
Widespread vaccination led the U.S. to declare measles eliminated from the country in 2000. Infected people have entered the U.S. from countries where measles is endemic and isolated outbreaks have sprung up periodically.
Measles is one of the most contagious viruses on the planet. But the measles-mumps-rubella vaccine against it is highly effective — about 95 percent effective in preventing infection.
When enough people are in a community are vaccinated against measles or have been previously infected with the virus, there's a protective effect called herd immunity that interrupts the spread of the virus to vulnerable people.