California is battling one of its largest measles outbreaks in nearly a decade after new cases in Sacramento pushed the state’s total to 39.
The outbreak, which began in Sacramento and Placer counties two months ago, has added 17 new cases — including four reported last week — state health officials confirmed Tuesday, the San Francisco Chronicle reported.
The incubation period for measles — the time between exposure and the onset of symptoms — typically ranges from 10 to 14 days.
Most people begin to show symptoms within seven to 14 days after exposure, though the full window can extend from seven up to 21 days.
Individuals with measles are contagious starting about four days before the rash appears and remain so until four days after it develops.
“With measles, this has been a significant year for us in that we are only a quarter of the way through the year and we already have 39 cases,” Dr. Eric Sergienko, chief of the California Department of Public Health’s communicable disease control division, said in a briefing.
Of the 39 cases reported this year, 85% are in people under 20. Nearly all — 95% — are either unvaccinated or have unknown vaccination status, with the latter group most likely unvaccinated.
Measles is a highly contagious viral respiratory disease marked by fever, cough, and a telltale rash, and it can lead to serious complications such as pneumonia or encephalitis.
Once widespread among US children — with an estimated 3 to 4 million cases each year before 1963 — the illness was declared eliminated in 2000 following the rollout of routine vaccination.
In recent years, however, cases have resurged as vaccination rates have declined.
California has logged 25 measles cases in 2025 so far, up from 15 in 2024 and four in 2023, after reporting zero cases in both 2022 and 2021. The state recorded four cases in 2020, according to official data.
As of April, 45 states in the United States have reported measles cases. The highest counts are in South Carolina with more than 997 cases, Texas with over 981, Utah with more than 597, and Arizona with at least 292.
The Centers for Disease Control and Prevention reports more than 2,200 cases and at least 48 outbreaks nationwide.
The first case in the Golden State was reported in an unvaccinated toddler who had recently traveled to South Carolina.
A measles outbreak is defined as three or more related cases. Earlier this year, California saw two such outbreaks — one in Riverside County involving three cases within a single family, and another in Shasta County linked to a church group with nine cases, according to Sergienko.
Before these incidents, the state had not recorded a measles outbreak since early 2020, when four cases were reported. The most recent major outbreak occurred in 2019, when 73 cases were recorded statewide, including 21 tied to a health care setting.
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