As the students went back to their homes, the virus spread to Brooklyn and Rockland County, then on to Ocean County and Orange County. In all, the outbreak lasted a year, and at least 3,502 people developed the disease.
When scientists analysed what had happened, they suggested that the chavrusa style of learning might have allowed for “particularly efficient transmission of mumps virus”. What might seem most surprising in this case is that the accidental super-spreader had received a full course of the MMR (measles, mumps and rubella) vaccine. It’s likely that he did have some immunity – like the other vaccinated children, he developed relatively mild symptoms with no complications – but he was still able to carry the virus and transmit it to others.
In fact, most vaccines don’t fully protect against infection, even if they can block symptoms from appearing. As a result, vaccinated people can unknowingly carry and spread pathogens. Occasionally, they can even start epidemics.Can you still transmit Covid-19 after vaccination? – BBC Future