California health officials discovered evidence of measles in routine wastewater testing as the state faces a sharp resurgence of the disease.
The Merced County Department of Public Health reported the finding, and while no confirmed clinical cases have been identified in the county, officials said it could indicate undetected circulation.
The state has confirmed infections climbing to 74 cases across seven counties, the highest annual total California has recorded in seven years.
Health officials said the jump is already far above last year’s numbers, with just 25 cases reported in all of 2025, underscoring how quickly the virus has regained ground.
Public health data indicates about 96% of those infected are either unvaccinated or have an unknown vaccination status, pointing to immunity gaps as the main driver of transmission.
The outbreak is unfolding even though roughly 95% of kindergarteners in California are vaccinated, a level typically considered sufficient for herd immunity.
Officials, however, warn that tightly clustered pockets of unvaccinated communities are allowing the virus to spread.
The resurgence is not limited to California. State health leaders say the situation reflects a broader national surge, with America experiencing measles activity at levels not seen in decades.
“The United States is experiencing the highest numbers of measles cases, outbreaks, hospitalizations and deaths in more than 30 years, driven by populations with low vaccination rates,” said California Public Health Officer Dr. Erica Pan in a statement.
Measles remains one of the most contagious viruses in the world.
Health experts note that in an enclosed space, a single infected person can transmit it to up to nine out of 10 unvaccinated people.
The virus can also linger in the air for up to two hours after an infected individual leaves, raising the risk of exposure for others entering the same space.
Public health officials are also warning about growing misinformation circulating alongside the outbreak.
During last year’s US measles surge, poison control centers reported a spike in calls involving children exposed to a widely available supplement.
Around the same period, online interest surged in whether certain vitamins could be used as alternative treatments.
A new study from Boston Children’s Hospital analyzed Google search trends for “vitamin A and measles” and “cod liver and measles,” finding increased public curiosity tied to those discussions.
The attention reportedly followed commentary from prominent figures, including Health Secretary Robert F. Kennedy Jr. and podcaster Joe Rogan, who discussed the nutrient as a possible aid against the virus.
Health experts stress that vitamin A and cod liver oil cannot prevent measles and are not substitutes for the MMR vaccine.
While high-dose vitamin A is sometimes used in hospital settings to treat complications in severely deficient or malnourished children, unsupervised use or supplementation can carry a risk of toxicity.
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