The threat of a measles epidemic has been drastically reduced since a vaccine for the illness was introduced in 1963, but San Mateo County could take some steps to reduce the possibility even further, according to a county Civil Grand Jury report.
Underlying continuing concerns is the fact that not everyone has been vaccinated against measles, the report said. As of July 11, 1,123 individual cases of measles have been confirmed in 28 states this year, which is the largest number of cases reported in the U.S. since 1992 and since measles was declared eliminated in 2000, according to the Centers for Disease Control.
"In an unimmunized population, a single case of measles can infect 15 to 20 additional people. High immunization rates therefore limit the chance of the contacts between infected and unimmunized individuals that can produce an outbreak," according to the Grand Jury.
Children are required to be vaccinated before entering preschool, kindergarten and seventh grade, and a vaccination rate of at least 93% is enough to achieve an acceptable level of immunity from an epidemic, according to public health officials.
However, vaccination rates for children dropped below this level in a few county schools during the 2017-18 school year, according to the report.
In the case of adults, people born before the vaccine was invented are immune because of exposure to the disease, and those born after were probably vaccinated during their school years, the report said.
Some residents grew up in foreign countries with vaccination requirements that were not as strict, but legal immigrants and green card holders must be vaccinated.
The presence of San Francisco International Airport and major transportation corridors in the county also increases the risk of infected individuals coming in contact with adults and children who have not been immunized, according to the report.
The San Mateo County Health Communicable Disease Control Program (SMCDC) isolates people who have come down with measles and tracks down those who have been exposed to the illness.
As a result, none of the four measles cases reported in the county last year, including three adults and one child under 18, spread to an individual who was not vaccinated.
"The fact that none of the four 2019 cases of measles spread within the county leads some public health officials to believe that pockets of adults who are not vaccinated may be sufficiently small and isolated that the risk of a large outbreak is not high," the report said.
To further reduce the risk, the Grand Jury is recommending that each school district report to its governing board when children who are overdue for a vaccination have been vaccinated.
It also recommends that the SMCDC test adults at county clinics for immunity to measles and offer to vaccinate them if their immunity level is low.
"The Grand Jury believes that measles testing in the clinics and follow-up immunization could identify and reduce adult pockets of individuals who are not immunized, thereby reducing the risk that they would present to the county," according to the report.
As a third step, the report recommends that the SMCDC add to its resources to strengthen the county's response to a potential measles outbreak.
The full report is available on the San Mateo County Civil Grand Jury website.