There were more than more than 32,000 cases of whooping cough in 2024, the highest tally in a decade. In California alone, the disease struck 2,000 people between January and October last year.
More than 60 infants younger than 4 months were hospitalized in the state. One died.
Whooping cough, or pertussis, is just the most stark example of what happens when vaccination rates decline. But it is far from the only one.
The pandemic interrupted childhood immunizations across the country, and rates have not yet recovered. As a result, hundreds of thousands of children are increasingly vulnerable to diseases once largely relegated to history books.
Most of them predominantly affect young children, like measles, mumps and rubella. But if immunizations continue to fall over the next few years — because of rising distrust, or more restrictive federal policies — preventable infectious diseases will resurface in all age groups, experts say.
“It might take a year or two, but there’s no question,” said Pejman Rohani, an infectious disease epidemiologist at the University of Georgia.
“We will have outbreaks,” he said.
It’s not just the unvaccinated who will have to worry. Even adults who were vaccinated decades ago may find themselves vulnerable to what are now considered childhood diseases.
Most people have forgotten the dangers of childhood diseases, said Dr. Alex Richter, a clinical immunologist at the University of Birmingham in Britain, where there are worrying upticks in measles and mumps.
Just a few decades ago, many children under the age of 5 died from infectious illnesses. Now children are more threatened by traffic accidents, drug overdoses and gun violence while disease has faded as a concern.
“That could all change if we don’t continue with vaccine policies,” Dr. Richter said.
High vaccination rates in a community protect not just the vaccinated, but also people who cannot receive some vaccines or who may not respond to them because of certain medical conditions, their age or weakened immune systems.
If fewer people are vaccinated, “we are making an active decision to make the world a less safe place for a significant proportion of the population,” Dr. Richter said.
For example, rubella, or German measles, can be dangerous for pregnant women and their babies. Yet pregnant women cannot be immunized against the disease because the vaccine contains a weakened live virus.
These days, they are typically not at risk, because there are fewer than a dozen rubella cases in the United States each year. That may change if vaccination rates drop. Worldwide, rubella is the leading vaccine-preventable cause of birth defects.
“If you’ve got nonimmune mothers catching rubella, then you have the lifelong complications of blindness and deafness and everything else,” Dr. Richter said.
Elsa Sjunneson knows that only too well. Her mother was infected with rubella during an outbreak in New York City in 1985 when she was pregnant, and Ms. Sjunneson was born with congenital rubella syndrome, or C.R.S.
In her case, that meant thick cataracts, hearing loss and a heart defect.
Before her first birthday, she had two surgeries that mostly fixed the heart defect, and seven eye surgeries that did not entirely restore her vision. She is blind in her right eye, has limited vision in the left and still needs hearing aids.
“I actually was really lucky — a lot of people who were born with C.R.S. didn’t survive,” said Ms. Sjunneson, who is a disability advocate and champions vaccination for rubella. “People don’t deserve to be exposed to diseases that can kill them.”
Anti-vaccine campaigns have often targeted the M.M.R. vaccine, which protects against measles, mumps and rubella. Experts tend to worry most about a resurgence of measles.
The virus is extraordinarily contagious, lingering in the air for up to two hours after an infected person has left the room. Each infected person can spread the virus to as many as 18 others.
The past offers a preview: In the late 1980s, budget cuts by the Reagan administration brought down vaccination rates, particularly among low-income Black and Hispanic children.
The fallout was swift. From 1989 through 1991, measles infected more than 55,000 Americans and killed 166.
Before the first measles vaccine was introduced in the 1960s, the disease killed an estimated 2.6 million people worldwide each year. The virus cripples immune defenses, leaving the body vulnerable to other pathogens.
A 2015 study estimated that before widespread vaccination, measles may have accounted for as many as half of all infectious disease deaths in children. Even now, the consequences can be serious. About 40 percent of people infected last year were hospitalized, according to the C.D.C.
Before the pandemic, immunization rates for M.M.R. and for pertussis held steady at roughly 95 percent, in part because of requirements for admission to public schools.
A drop during the pandemic was not surprising. But even as society has returned to normal, vaccination rates have continued to decline, dipping below 93 percent nationwide for the 2023-24 school year.
That means about 280,000 school children remain susceptible to these diseases, raising the risk of outbreaks in schools and other public spaces.
Unvaccinated adults are at risk, of course, but so are those who do not mount an adequate immune response to vaccines or who received only a single dose.
And there is another unexpected consequence to declining vaccination rates.
The immunity induced by some vaccines can wear off over the decades. The decline means that if outbreaks were to occur more often, even vaccinated adults might be vulnerable to certain illnesses.
In rare cases, for example, immunity gained from the measles vaccine may wane. Of the 284 measles cases recorded among Americans last year, 11 percent were in people who had received one or two doses of the vaccine.
That may help explain why 27 percent of cases were adults older than 20.
“We’ve now moved away from a time when measles was only in children,” said Alexis Robert, a research fellow in infectious disease modeling at the London School of Hygiene and Tropical Medicine.
Immunity against mumps also may decline. Although vaccination overall has decreased the number of mumps cases by 99 percent, there have been outbreaks in schools and universities, where students have close, prolonged contact.
Mumps is often a mild condition in children, but it can sometimes cause fertility problems in boys and severe complications in adults.
The illness may be mistaken for a typical respiratory infection at first, but it can bloom into a painful, full-body “100-day cough.” Each bout of coughing ends with a whooping sound, and may result in vomiting, cracked ribs and difficulty breathing.
Decades ago, the vaccine relied on whole cells from the bacteria that cause whooping cough. It was potent but harsh, often setting off high fevers and seizures.
“There’s no way, I mean absolutely no way, that parents would tolerate that sort of reaction currently,” said Dr. Kathryn Edwards, a vaccine expert who has studied pertussis for 40 years.
A newer version of the vaccine, introduced in the 1990s, is much easier on the body. In most people, this formulation provides decades of protection against severe disease.
But the new pertussis vaccines do not fully prevent infection, and sometimes, the protection wears off.
Experts now believe this is one reason that more adolescents than young children have become infected with whooping cough during outbreaks in recent years.
“That was really the first hint” of declining vaccine immunity, Dr. Edwards said. The C.D.C. now recommends a booster dose for adolescents.
If the vaccination rates were to fall to 75 percent in the next few years, older adults who received the original vaccine might still be protected.
But people who were never immunized or adults who received the newer vaccine as children might be susceptible.
According to epidemiological modeling by Dr. Rohani and his colleagues, cases would rise most dramatically in infants — who are too young to be fully vaccinated — and in children ages 5 to 15.
School-aged children tend to have the most contacts, so they are the “core transmission groups,” Dr. Rohani said.
He and other experts said they hoped vaccination rates would decline slowly, but worried about the consequences.
Vaccines are always a tougher sell than treatments, because they are given to healthy individuals, Dr. Richter said.
In the extremely rare cases when someone experiences a serious side effect, it can be catastrophic.
“All you need is one or two of those stories to have a massive impact on vaccine takeup,” she said. “This is where you have a tension between community and individual.”
