Recent changes to federal vaccine policies could cause a resurgence of some serious diseases, medical experts warn. Here’s a primer on why the diseases are so dangerous and how the vaccines are so effective.
In January, the US Department of Health and Human Services (HHS) announced significant changes to the childhood vaccine schedule, a longtime goal of HHS Secretary Robert F. Kennedy Jr., and other vaccine skeptics. The new federal guidelines removed six widely-used and effective vaccines from the list of universally recommended inoculations, including the seasonal flu shot, and designated them either only for high-risk individuals or to be given after consultation with a doctor.
The vaccine schedule doesn’t demand anyone get a vaccine. Nor does it prevent anyone from getting one. But the official guidelines have historically informed insurers which vaccines they must cover, and they set the tone for public confidence in vaccines overall. The changes, which were made with little scientific review, have moved vaccine hesitancy from the fringes directly into the federal government’s health infrastructure.
Under the new federal vaccine schedule, shots for RSV, Hepatitis A and B, and meningococcal disease are suggested only for “high-risk groups.” The rotavirus vaccine and seasonal flu shots are now considered entirely optional.
Most medical institutions protested the changes and said they will continue recommending the old schedule, but they fear that loosening the federal guidance on vaccines will lower vaccine rates even further, giving serious but vaccine-preventable diseases the momentum they need to come roaring back.
But what are these diseases, and what do they do to the body?
Here is a quick primer on the diseases that could be ready to strike the United States again now that their vaccines are no longer universally recommended by federal health officials.The timing and dosage is different for each vaccine, so make sure you talk to your doctor for specifics.
Measles remains one of the universally recommended vaccines, but since falling vaccination rates have already spurred the biggest outbreak in decades, and outbreaks of one disease can make others more likely, let’s start there.
Measles
What is it? Virus
Why is it dangerous?: Measles starts out with flu-like symptoms: a cough, a runny nose, a high fever. But after four days or so comes the tell-tale rash, first on the face and neck, then all over. The virus can cause pneumonia. It can cause encephalitis, or inflammation of the brain. In about one out of every 1,000 cases, this can result in permanent brain damage. One to three of every 1,000 cases is fatal.
Like long COVID, measles can also result in serious complications years after infection. The virus can infect the central nervous system of your 1 year old and then kind of hide out there before expressing itself again even a decade later with a rare but degenerative disease that is almost always fatal.
Measles also can ravage your immune system, stealing its ability to fight other and future infections.
How contagious is it? Measles is perhaps the most contagious pathogen humans have ever encountered, doctors say. If you have measles and enter a room with 10 other unvaccinated people, you will infect 9 of them.
How does it spread?: It’s airborne, and spreads by coughs and sneezes. The virus can linger in droplets in the air for two hours after the infected person leaves the room.
How effective are the vaccines? The measles vaccine, currently given in two doses along with two other vaccines, is 97% effective after the second dose. According to several data sources, more than 94% of cases in the current outbreak are in unvaccinated people or whose status is unknown. Only 3% are in people who had only one dose at the time of infection.
The first measles vaccine in the US became available in 1963. According to the Children’s Hospital of Philadelphia’s Vaccine Education Center (VEC), before the vaccines, 3 to 4 million people got measles every year, 48,000 people were hospitalized, and 500 people died. In the year 2000, the year measles was declared eliminated in the United States, there were 86 confirmed cases, and just 1 recorded death.
How safe are the vaccines? All vaccines carry some risk, but side effects are rare, and serious side effects are super rare. With most vaccines, any serious side effect is also a side effect of the disease itself, but with the disease the side effects are far more common and more severe.
With the measles vaccine, the most common side effects are a sore arm or small rash at the injection site, and temporary fever or pain in the joints.
“About 1 of every 3,000 to 4,000 children will develop a fever that increases rapidly and causes a fever-associated seizure,” the VEC says. “This is called a febrile seizure. While febrile seizures are scary, they do not cause long-term harm.”
Rotavirus
What is it? Virus
Why is it dangerous?: Rotavirus is rarely fatal, even among the unvaccinated, but it’s far from gentle. It infects infants and toddlers mostly, attacking their intestines and causing severe diarrhea and vomiting, usually all at once. Severe dehydration is the biggest concern, and it can be difficult to get under control without hospitalization.
How contagious is it? Extremely contagious.
How does it spread?: By swallowing tiny bits of poop, which is gross but pretty common among young children who may not yet be potty trained, and need help washing their hands.
How effective are the vaccines? According to the Cleveland Clinic, about 70% of the children who receive the vaccine don’t get the virus at all, but like most vaccines, the shots are even better at minimizing the symptoms of infections.
“90% of vaccinated children either get protected from severe rotavirus or avoid the disease altogether,” the Cleveland clinic says.
How safe are the vaccines? In a sign that the vaccine safety system works, the first rotavirus vaccine was pulled from the market about a year after it was introduced in 1998 after rare reports of intestinal blockage. About 1 in every 10-to-30,000 children who got the vaccine developed the blockage.
The two current vaccines underwent significant testing before they were approved in 2006 and 2008. Because of the issue with the previous version, scientists vastly increased the size of the clinical trials for the new vaccines, the Vaccine Education Center says, following up with more than 130,000 babies.
The vaccines were given to “millions and millions of children,” and subsequent studies found that some cases of intestinal blockage were still possible, but in far fewer numbers than the previous vaccine. And, the studies found, the rotavirus itself can cause intestinal blockage at about the same rate as the new vaccines.
While rotavirus causes intense vomiting, diarrhea, fever, and loss of appetite, the vaccines do not.
Respiratory syncytial virus (RSV)
What is it? Virus.
How is it dangerous?: In adults and healthy kids, RSV can present like a regular cold. But it can be very serious for young children, children with weakened immune systems, older people, and premature babies. RSV is the leading cause of hospitalization in infants. RSV is super common and normally mild, but unlike a common cold, it can also cause pneumonia and other severe respiratory distress.
How contagious is it? Very.
How does it spread?: Droplets in the air from coughing and sneezing. It can also spread through touching contaminated objects.
How effective are the vaccines? There are a few options for RSV prevention, but they are still babies themselves, only coming on the scene in 2023. One vaccine is intended for older people; another vaccine is tailored for pregnant women, and it also protects the baby for the first 6 months of life; and finally, babies can be given monoclonal antibodies immediately after birth. In most cases, if the mother gets the vaccine during pregnancy, the antibodies are not necessary.
Recent studies have shown that vaccination during pregnancy and the antibodies are each highly effective. The vaccine protects about 6 in 10 infants from severe disease and the antibodies protect 8 in 10.
How safe are the vaccines? Side effects to the antibodies are rare and mild. Only 1 in 100 babies even develops a minor rash. During the extensive clinical trials for the vaccine given during pregnancy, slightly more vaccinated women gave birth prematurely than in the control group. So the Food and Drug Administration approved it for a little later in the pregnancy, which brought the increased risk back in line with the overall risk of premature birth in any pregnancy.
Hepatitis A
What is it? Virus
How is it dangerous?: Hepatitis A attacks the liver, causing jaundice, vomiting, diarrhea and fever. It does not cause long-term complications like its cousin Hepatitis B, but it’s still not great. The United States sees about 1,000-17,500 cases of hepatitis A virus every year.
Some 75 people die every year in the United States from severe hepatitis A cases.
How contagious is it? Very.
“Unfortunately, people infected with hepatitis A can transmit the virus to others up to two weeks before they have symptoms, so they may be infecting others without even knowing they have hepatitis A themselves,” the Vaccine Education Center says.
How does it spread?: By eating or drinking contaminated food or beverages, or not washing your hands after changing the diaper of an infected baby.
How effective are the vaccines? If all vaccines are like the Avengers, the Hepatitis A vaccine is Captain America. One dose of the vaccine protects more than 95% of adults and more than 97% of children. Two doses provides nearly 100% protection. And that protection can last up to two decades.
How safe are the vaccines? Side effects are rare and minor, but are most commonly soreness or redness at the injection site. About 5 in 100 people can have brief headaches afterward.
Before the vaccine: The first vaccine was approved in 1995, but it was not universally recommended until 2006. Since then, hepatitis A cases in the U.S. dropped by 95%.
Hepatitis B
What is it? Virus
How is it dangerous?: Hepatitis A may not cause long-term complications, but Hepatitis B most certainly does. It is known as the “silent epidemic,” because most of those complications can develop decades later in someone who never even knew they were infected. Hepatitis B often causes Cirrhosis, a permanent liver problem, and is the cause of nearly half of all cases of liver cancer.
How contagious is it? Hepatitis B most often spreads among adults through unprotected sex or sharing needles and other drug paraphernalia. But the virus can teem inside the tiniest specks of blood and can also live on objects for a long time, making it easy to spread among team locker rooms and for infected parents to pass to their infants. Shared toothbrushes or razors can be as much a problem as a shared needle.
“It’s estimated that about three-quarters of a million to 2 million people in the U.S. are chronically infected with hepatitis B virus,” the VEC says.
How effective are the vaccines? The new vaccine schedule recommends the Hepatitis B vaccine only for high-risk groups. The problem is that this strategy already failed in the early 80s. When the vaccines were first introduced, they were also aimed only at high risk groups.
“Because the disease can be transmitted to those who are not in high-risk groups, this vaccine strategy didn’t work,” the VEC says.
But in 1991, the vaccine was recommended for all infants and young children, and then boom went the dynamite.
“This strategy has virtually eliminated the disease in children less than 19 years of age.”
How safe are the vaccines? The most common side effects of the vaccine are pretty standard: soreness at the infection site, maybe a low grade fever.
In “extremely rare” cases, the VEC says, people who get the vaccine can develop a severe allergic reaction, Anaphylaxis, within 15 minutes of getting the shot, which is why medical experts recommend people who get the vaccine hang out a little in the doctor’s office or pharmacy before leaving.
In this case, “extremely rare” means 1 out of every 600,000 doses. The virus itself however is far worse. According to the CDC, 90% of infants who get the virus will later develop chronic liver infections. As much as 25% of all chronic infections will lead to serious liver disease, including liver failure or cancer.
The flu
What is it? You know what the flu is. You probably got it over the holiday break.
How is it dangerous?: The flu, which has been brutal the last two seasons, kills tens of thousands of people every year, most of them unvaccinated. More than 280 children died of flu complications in the United States last flu season, the North Carolina Health Department says, the highest number in 15 years.
As of Feb. 6, 2026, there were 236 flu-related deaths this flu season in North Carolina, including five children. The current flu season has been so bad in North Carolina that several hospitals limited visitors and implemented mask requirements.
“Influenza typically hospitalizes and kills more people in the U.S. than any other vaccine-preventable disease” The VEC says.
How effective are the vaccines? Flu shots are probably one of the biggest sources of confusion and medical misinformation out there. Flu shots are intended to be as seasonal as the flu itself, meaning each year’s shot is tailored to the particular strain of flu expected to dominate the season. But while many vaccines are excellent at stopping infection entirely, flu shots are less about keeping the flu out of your body and more about keeping you out of the hospital.
Even if a given year’s shot is not a direct match to the dominant strain, it still provides ample protection against the worst case symptoms.
According to the American Medical Association, flu shots in the 2022-23 season prevented 6 million flu-related illnesses, nearly 3 million doctors visits, 65,000 hospitalizations, and 3,700 deaths.
How safe are the vaccines? Side effects from the flu shot are rare. Sore arm, low-grade fever, maybe 24 hours of feeling a little run down. Severe allergic reactions to the shot occur in approximately 1 in a million people.
There are indications that in “very rare cases,” flu vaccines can lead to Guillain-Barré syndrome (GBS), a condition in which the body’s immune system starts attacking the nervous system. But as with most serious side effects to a given vaccine, the disease itself is far worse.
The flu causes GBS about 17 times more often than vaccination, the Vaccine Education Center says.
Meningococcus
What is it? Bacteria.
How is it dangerous?: This is another scary one. Meningococcus, which can lead to sepsis (blood poisoning) and meningitis (the swelling of the brain lining) is “one of the most rapid and overwhelming infectious diseases known to man,” the VEC says.
Not all Meningococcus infections will develop these serious symptoms, but when they do, they’re deadly. Between 10 and 15% of meningitis cases associated with infection are fatal. Some 40% of sepsis cases are fatal.
And the danger comes fast. Many deaths tied to sepsis occur less than 12 hours after the first symptom.
“This is what meningococcal disease does. It strikes healthy people without warning: infants, teenagers, young adults. Early symptoms mimic the flu. By the time the characteristic rash appears, the infection may have already caused irreversible damage,” Dr. Jake Scott, an infectious diseases specialist at Stanford’s School of Medicine, wrote in a recent blog post.
Nearly 20% of survivors will face long-term dangers, including kidney disease, intellectual disability, a loss of hearing, sudden seizures, and even amputation.
Meningococcus “primarily targets children younger than 1,” the VEC says.
How contagious is it? Very. It spreads through close contact and is especially contagious in day cares. Though it mostly affects young children, it can also spread easily among teens and young adults in dorms and other crowded, close quarters.
How effective are the vaccines? There are several subgroups of the bacteria and a few vaccine options depending on the group. But the bottom line is that all the vaccines are effective against the specific bacteria they target. In total, the vaccines are about 85% effective in preventing infection and even better at minimizing complications.
How safe are the vaccines? “The meningococcal vaccines may cause pain or tenderness where the shot is given, but do not cause any serious side effects,” the VEC says.
