Vaccinations urged as dangerous strain of virus emerges in Northern California

A potentially more dangerous strain of the mpox virus has been confirmed in Northern California, prompting officials to urge vaccinations to residents of the Golden State.

The disease was formerly known as monkeypox, but the World Health Organization phased out that name in 2022 in favor of mpox because of “racist and stigmatizing language” online during an earlier outbreak, the agency said.

A case of the clade I strain of the virus was confirmed for the first time in San Francisco on April 14, the San Francisco Department of Public Health said in an April 16 news release.

The strain is considered potentially more dangerous than the somewhat more common clade II strain, health officials said.

An unvaccinated adult who had close contact with someone who had recently traveled internationally had to be hospitalized with the virus, the health agency said.

The agency advised those at risk for mpox to seek vaccinations, but said the risk is low for those who are not at risk.

It is the seventh clade I mpox case in California, the Los Angeles Times reported. Previous cases were reported in Los Angeles and San Mateo counties.

Clade II mpox has been circulating at low levels in California since 2022, the California Department of Public Health said.

The agency reported 7,349 cases of mpox from both strains in California since 2022. About 350 people have required hospitalization.

“While clade I mpox cases remain rare in the United States, clade II mpox cases continue to occur in San Francisco and throughout California,” said San Francisco Health Officer Dr. Susan Philip. “This development reminds us just how important it is to be fully vaccinated against mpox if you are at risk. With summer travel and events quickly approaching, now is a great time to seek the mpox vaccine.”

Here’s what to know:

What is mpox?

Mpox is an illness caused by the monkeypox virus, which comes in two strains called clades, the Mayo Clinic said. It can be spread to humans by animal hosts, such as rodents or monkeys, or between people.

The clade I strain causes more serious illnesses and is linked to more deaths. It also spreads more easily, WHO said. The clade II strain causes less serious illness and has a lower chance of death.

The virus was discovered in monkeys kept for research in Denmark in 1958, the World Health Organization said. The first reported human case took place in central Africa in 1970.

Mpox has been reported sporadically in central, east and west Africa, the agency said. In May 2022, a global outbreak of the clade II strain spread to the United States.

How common is mpox?

The virus is currently considered rare in the United States, with both clades spreading at low levels, the U.S. Centers for Disease Control and Prevention said.

Since 2022, there have been 114,000 clade II monkeypox cases worldwide, with more than 220 reported deaths, the agency said. Since 2024, more than 53,000 confirmed cases of clade I mpox have been reported globally, including more than 150 deaths

Who is at risk for mpox?

“While anyone can get mpox, most reported cases in the United States are among gay, bisexual, and other men who have sex with men, as well as transgender persons who have sex with men,” the San Francisco Department of Public Health said.

People at higher risk for serious illness or death from mpox include people who have a seriously weakened immune system, children younger than age 1, people who have had eczema and pregnant people, WHO said.

In the U.S., you might be at higher risk of mpox if someone you’ve had sex with in the past two weeks has been diagnosed with mpox or you’ve had sex at a sex club or bathhouse, the Cleveland Clinic said.

Your risk also may increase if you’ve had sex at an event or location where mpox is spreading, you’ve been diagnosed with a sexually transmitted infection in the past six months or you’ve had sex with more than one person in the past six months, the clinic said.

What are the symptoms of mpox?

People usually get sick within 21 days of exposure to the mpox virus in animals or people, the CDC said. During this time, the patient has no symptoms.

People with mpox often have flu-like symptoms for a few days before developing a rash on their hands, feet, chest, face, or mouth or near the genitals, the agency said.

“The rash can look like pimples or blisters at first, and they may be painful or itchy,” the CDC said. The rash goes through several stages.

In some cases, patients have reported feeling pain around the anus or the need to poop, the Mayo Clinic said.

What are the complications of mpox?

Possible complications of mpox include bacterial infections, which can lead to sepsis, loss of vision or eye damage from mpox ocular infections, myopericarditis (inflammation around your heart), encephalitis (inflammation in your brain), pneumonia, respiratory distress (extreme difficulty breathing and getting oxygen to your body) or severe scarring, the Cleveland Clinic said.

When should you see a doctor about mpox?

Talk to your doctor if you have mpox symptoms or have been exposed to someone with mpox, the Cleveland Clinic said.

Seek emergency medical care if you have trouble breathing or severe shortness of breath, new or worsening chest pain, neck stiffness, confusion or difficulty thinking clearly, difficulty speaking or moving, loss of consciousness, seizures or a wound that’s not healing or is very painful.

How does mpox spread?

“Mpox mainly spreads through sex or close contact to a person who has mpox infection,” the California Department of Public Health said.

Direct contact with the rashes, scabs or body fluids of a person with mpox, close, face-to-face contact, intimate activities and sexual contact, and contact with fabrics or objects that have touched the rashes or body fluids of a person with mpox also can spread the virus, the Mayo Clinic said.

A pregnant person can pass the virus to the unborn baby, and the virus also can spread through close contact with an infected animal, the clinic said.

How can you prevent mpox?

The mpox vaccine is the best protection against both strains of the virus, the San Francisco Department of Public Health said.

“It significantly reduces the chance of infection, and it helps prevent severe illness, hospitalization, and death among people diagnosed with mpox,” the agency said.

The department also encouraged condom use and reducing the number of sexual partners to decrease the risk of mpox.

The vaccine requires two doses, the CDC said. It can potentially be effective even after you’ve been exposed.

Avoid touching rashes and avoid contact with animals where mpox is common, the CDC said. Also avoid contact with contaminated objects and materials.

How do you treat mpox?

Most people will recover from mpox with pain management and supportive care, the CDC said.

There’s no formal medical treatment for mpox in the United States, although some smallpox antivirals may be effective, the National Institute of Allergy and Infectious Diseases said.

Do not pop blisters or shave areas with blisters, WHO said.

How can you tell mpox from other illnesses?

Lab tests may be needed to tell mpox from chickenpox, measles, bacterial skin infections, scabies, herpes, syphilis, other sexually transmitted infections and medication-associated allergies, WHO said.

“For these reasons, testing is key for people to get care as early as possible and prevent severe illness and further spread,” the agency said.