During the first national measles outbreak since the U.S. declared the disease eliminated in 2000, Stanford professors have expressed cautious optimism — while emphasizing the need for prevention.

In 2025, the U.S. recorded its highest number of measles cases in decades. 1,136 confirmed cases have already been reported nationwide as of Feb. 26, nearly half of 2025’s record-setting total.

In January, 14 measles cases were reported across six counties in California, including Napa and San Mateo, all tied to a single traveler. In February, 12 additional cases emerged in the Shasta and Santa Clara counties. Despite these clusters, officials clarify that there is not currently a widespread outbreak in the Bay Area.

“So far, these cases have not represented sustained community spread of a single domestic strain,” said Hayley Gans, clinical professor of pediatrics and infectious diseases at Stanford University School of Medicine. However, she cautioned that if a single strain were to circulate within the country for more than 12 months, the U.S. could lose its elimination status — a situation that she said has a “high risk” of occurring.

Measles is “probably the most infectious disease that we know of in humans,” said Yvonne Maldonado MD ’81, a professor of global health and infectious diseases. Maldonado ascribed measles’ infectiousness to its mode of transmission — the virus spreads through tiny airborne droplets that can linger in the air for up to two hours after an infected person leaves a room.

Because of its high transmissibility — nearly five times that of COVID-19 — measles requires very high levels of community immunity to prevent outbreaks. One dose of the measles vaccine provides about 95% protection, Maldonado said. A second dose increases protection to approximately 97%.

“You need about 95% of people in a community to be immune to prevent an outbreak,” Maldonado said.

While California has reported cases in multiple counties, Maldonado emphasized that this does not necessarily mean there is a widespread outbreak in the Bay Area.

In epidemiological terms, an outbreak is typically defined as three or more related cases of a disease, she added. “Over 95% of the cases that we’ve seen so far in the U.S. are people who just didn’t get vaccinated,” Maldonado said.

“Stanford students are required to be vaccinated for measles and two doses of measles vaccine provide approximately 97% protection, and any breakthrough cases are typically mild,” Vaden Health Center Medical Director Robyn S. Tepper wrote in an email to The Daily. Tepper said that requiring students to provide vaccine documentation before enrolling helps defend the Stanford community against measles.

Tepper added that students should ensure their family members are also vaccinated and stay up to date on routine immunizations. For those traveling internationally, she recommends consulting a travel clinic, as additional vaccines may be required.

“Stanford is not going to be isolated because of the global nature… with people leaving and coming onto campus, or visitors coming, or some of the increased possibility for exposure” Gans said, noting that Stanford is a popular tourist destination.

“We remain committed to supporting a healthy campus and will continue to monitor public health alerts,” Tepper wrote.

Overall, the experts emphasized the value of prevention. “The vaccine is really our only way of protecting people,” said Gans.