San Francisco on Monday reported its first confirmed measles case since 2019, in an infant who had traveled abroad. According to the Department of Public Health (opens in new tab), the patient was under the age of 1, when children typically get their first vaccination for measles.

Surging measles cases have created a public health crisis across the country, with outbreaks in hot spots like Utah and South Carolina, largely tied to low vaccination rates. In California, 39 cases have been confirmed by public health officials. 

To learn more about the risk to the public and how to stay protected from measles, we spoke with Dr. Peter Chin-Hong, an infectious disease expert at UCSF. 

According to Chin-Hong, the high rate of vaccination in San Francisco provides a strong level of herd immunity to the vulnerable, but certain at-risk populations can go further to guard against infection.  

This interview has been edited for length and clarity.

We’ve been hearing about measles outbreaks in California for a few months. With the infection in San Francisco, what’s your level of concern?

From a health perspective, I’m not worried, because the Bay Area — and San Francisco in particular — has one of the highest vaccination rates in the country. This case looks like what we used to see before last year: Someone traveled abroad, contracted measles, and came back. It didn’t originate here.

It’s not worrisome because of herd immunity. With our vaccination rates, protected people essentially act as a force field. A small number of people are severely immunocompromised — even if they were vaccinated, they may not have the antibodies to protect themselves. But the rest of the population does.

How does that compare to what’s happening in other parts of the state and country?

In Sacramento and Placer counties, you have a growing outbreak because there’s a pocket of vulnerability. In South Carolina, Utah, Arizona, and West Texas, maybe only 40% to 60% of children are vaccinated. That leaves a lot of people unvaccinated and exposed — and measles can spread like wildfire.

With high vaccination rates in San Francisco, the chance of someone with measles coming into contact with someone who isn’t protected must be low. Is that how herd immunity works?

Yes, exactly. Say you, for some reason, didn’t get vaccinated — but everyone in your household and your workplace did. They wouldn’t be able to pass it on to you. But where more people are unvaccinated, the chances of an unvaccinated person coming into contact with that baby, for example, and then spreading it to someone else who’s unvaccinated is much higher. Every person starts a chain reaction. That’s what’s happening in South Carolina and West Texas.

Measles is one of the most contagious infectious diseases for the unvaccinated. How does the virus spread?

It’s airborne. Have you seen “The Last of Us”? It’s the same idea, although that’s a fungus and this is a virus. The problem is that respiratory droplets linger in the air. Someone could have measles, come into a room, and then leave — and if you’re unvaccinated, you could walk into that room two hours later and still catch it. Nine out of 10 unvaccinated people will contract measles if they share a space, or even a previously occupied space, with an infected person.

What are some of the initial symptoms? How does it differ from something like a cold?

In the beginning, it’s hard to tell. That’s part of what makes it a scary disease — it’s so transmissible, and it might just seem like a cold or a cough. But three to four days later, you’ll get a full-body rash. That’s when people might realize something is up. There are also more serious complications: One in 20 people gets pneumonia, and one in a few hundred gets a brain infection and inflammation. Last year, there were three deaths — two children and one adult. That was the first child death [from measles] in 20 years and the first adult death in 10.

In 2000, the U.S. declared measles eliminated. Is that status at risk?

The loss of elimination status is mainly a symbolic marker, but it’s also a metaphor for where we already are. Canada has lost it. It’s kind of embarrassing, really, for the U.S., because we traditionally had one of the best public health systems in the world. We haven’t officially reached that threshold yet, but we’re probably already there.

The relevant history in California is the Disneyland outbreak from several years ago. A child with measles went to Disneyland and infected a number of other people, triggering a large outbreak across different communities — particularly in places like Marin. It exposed pockets where parents weren’t vaccinating their children, and it led to stricter state laws around who could be exempt from school vaccine requirements.

What is the internal protocol at a hospital like UCSF if you have a child or adult with a suspected measles infection?

A common scenario now is that a child has measles but doesn’t realize it and goes to the emergency room. A number of people could pass through that same space while the child is there. Public health officials then go in and determine who was present during that window — who has documented immunity or proof of vaccination. Anyone without it has to stay home for three weeks, because measles can take that long to incubate.

That’s not five days like COVID. If we thought COVID was disruptive to schools and daily life, measles takes it to another level.

Should older adults consider getting booster vaccines before traveling?

In general, if you’ve been vaccinated, it usually lasts a lifetime. People born before 1957 or so are fine — back then, almost everyone contracted measles naturally and developed immunity. If you were born before 1968 or so, you may have received a less effective vaccine. Those individuals should talk to their provider about getting an antibody test and, if needed, an updated vaccine.

What about infants and young children?

The standard vaccination schedule is at year one and year five — before kindergarten. But what about younger babies traveling to outbreak areas or going abroad, like the case we saw in San Francisco? They wouldn’t normally receive the vaccine on schedule. However, there is a guideline for travel: If a baby is between six months and one year old, an early dose is available. Under six months, though, a vaccine isn’t an option — the infant’s immune system isn’t yet mature enough to respond. That’s a vulnerable window.

What’s your advice for parents of children under one year old?

In the Bay Area, it’s really safe — I wouldn’t change any behavior. California broadly is in good shape.

If you’re going abroad, though, talk to your provider about vaccination even for destinations you might not flag as high-risk. The U.K., for example, has significant measles activity right now. People don’t usually think of it the way they might think of traveling to India or Indonesia, but sometimes the risk is where you don’t expect it.

Domestically, if you’re taking an infant to an outbreak area like Utah, South Carolina, or West Texas, treat it almost like going abroad.

Among vaccinated people, who remains most vulnerable?

The truly high-risk group is actually quite narrow. It’s not a broad category the way people sometimes thought about immunocompromised individuals during COVID. For measles, the most at-risk are people taking certain cancer medications that deplete antibodies, or those who have recently received a transplant. Those people would know who they are. It’s not someone with diabetes, or even someone with HIV who has a high T-cell count.