Mpox Virus Detected in Bay Area Wastewater; Officials Recommend Vaccination

Health officials in California continue to detect the mpox virus in Bay Area wastewater and are recommending vaccination for individuals at higher risk of exposure.

This follows the identification of a more severe strain of the virus, Clade I, in several California residents, including a recent case in San Francisco. While Clade II remains the predominant strain in the U.S., officials report current case rates are higher than in recent years. The available vaccine is effective against both known strains.

Current Detection and Case Data

Wastewater data indicates that Clade II of the mpox virus continues to be occasionally detected in the Bay Area. Since the initial 2022 outbreak, San Francisco has recorded over 1,000 cases of Clade II mpox, with current data showing an average of fewer than two new cases per week statewide.

However, health officials report that the overall Clade II case rate in California is currently higher than in the previous two years. The current weekly case rate is 14.5, compared to 3.4 for a comparable period in 2025 and 5.8 in 2024.

A different strain, Clade I, has been identified in seven individuals in California. The first case was detected in San Mateo County in November 2024, with subsequent cases in Los Angeles County in October 2024 and January 2025. The most recent case was identified in a San Francisco resident who was hospitalized. The individual was unvaccinated and had close contact with an international traveler.

Virus Strains and Transmission

Mpox is caused by a virus that spreads through close, usually skin-to-skin contact. Transmission can occur through:

Direct contact with infectious rashes or scabs.
Intimate physical contact, such as kissing or sex.
Contact with infected bodily fluids or contaminated items.

Health officials state that Clade I infections may cause more severe illness and spread more easily through close personal contact than the Clade II strain, which was responsible for most U.S. cases starting in 2022.

The Centers for Disease Control and Prevention (CDC) notes that while initial Clade I cases outside Africa were linked to travel, by late 2025, cases were reported in central Western European countries among individuals with no documented international travel history. The CDC concluded these cases were likely related to intimate or sexual exposure.

Vaccination Recommendations

The Jynneos vaccine, available in the U.S., is a two-dose series administered approximately one month apart. Maximal immunity develops two weeks after the second dose. The vaccine provides protection against both Clade I and Clade II mpox.

The CDC and California health officials recommend vaccination for the following groups:

Gay and bisexual men and other men who have sex with men.
Transgender, nonbinary, or gender-diverse people who, in the past six months, have had multiple sexual partners or a new sexually transmitted infection (STI) diagnosis.
Anyone who has had sex at commercial sex venues or plans to attend events where such activity may occur.
People living with HIV.
People taking HIV pre-exposure prophylaxis (PrEP) or who are eligible for it.
Sex workers.
People traveling to countries with Clade I outbreaks who anticipate sexual contact with new partners.
Individuals with a sex partner in any of the above categories.
Certain laboratory workers.

In 2024, the CDC updated its recommendations to include travelers to Clade I outbreak areas who anticipate specific activities. The vaccine was originally approved for people aged 18 and older, but in 2022, the U.S. Food and Drug Administration issued an emergency-use authorization allowing providers to administer it to people under 18 determined to be at high risk of infection.

Symptoms and Public Guidance

Symptoms of Clade I and Clade II mpox are similar and often begin with flu-like conditions, such as fever, chills, headache, muscle aches, or swollen lymph nodes. The virus typically causes a rash, sores, or spots resembling pimples or blisters that can occur anywhere on the body. These spots can progress from red, flat spots to bumps that fill with pus and eventually form scabs. The incubation period ranges from 3 to 17 days.

Health officials state mpox is unlikely to spread through casual contact in settings like planes, offices, or stores.

Public guidance includes:

Individuals with symptoms should contact a healthcare provider for testing.
Those exposed to mpox should get vaccinated before symptoms develop.
Infected individuals should stay home and avoid contact with others until the rash has fully healed.

Vaccine Access

Individuals with regular healthcare providers are advised to inquire about vaccination through them. Vaccines are free, though copays may apply for the provider visit.
Health insurance should cover vaccine costs for eligible individuals per CDC recommendations.
Those without regular providers or insurance can visit public health clinics, with appointment or walk-in options available.
Vaccination does not affect immigration status or processes, and immigration status is not required to receive a vaccine.
Some pharmacies offer mpox vaccination appointments, though individuals should verify insurance coverage as out-of-pocket costs may be significant.