The United States is on track to experience the largest outbreak of measles since health officials declared the disease eliminated from the country in 2000. This year alone has seen more than 1,000 cases reported across several states, with severe outcomes for young children: Many have been hospitalized, and two children have died.

About half of parents surveyed by the Kaiser Family Foundation say they’re worried about measles. Yet as cases continue to spread, so does conflicting information about vaccine safety and effectiveness. Much of this information is false or misleading, but the volume can make sorting fact from fiction difficult.

Bonnie Maldonado, MD, the Taube Professor in Global Health and Infectious Diseases and a professor of pediatrics, has spent 15 years researching measles and vaccine effectiveness in babies and recently co-authored a publication forecasting millions of U.S. measles cases over the next 25 years if vaccination rates continue to decline.

We asked her to address the risks of measles, vaccine safety and effectiveness, and other common points of confusion. These are the five main takeaways.

Complications are more common than you might think.

While measles is sometimes compared to chickenpox, it is much more likely to lead to severe health complications and even death.

“Measles works by invading the respiratory tract, where it infects local lymph nodes before eventually spreading to the rest of the body,” Maldonado said. For this reason, measles tends to impact a person’s respiratory tract, brain, and other parts of the central nervous system.

While measles can lead to relatively benign symptoms such as diarrhea and ear infections, it also causes pneumonia in about 6% of healthy children and sometimes more frequently among malnourished children. More rarely, it can cause encephalitis, or swelling of the brain. Measles can also suppress the immune system, making children vulnerable to other diseases.

The very young and old are most likely to have complications, along with pregnant women and people with compromised immune systems, such as those with HIV. During the current U.S. measles outbreak, nearly one in five children under 5 with measles has been hospitalized. In low-income countries where children are more likely to suffer from poor nutrition and have limited health care access, the death rate can be very high – 4-10%, Maldonado said.

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Measles is one of the most infectious diseases on the planet.

Most respiratory viruses spread through large droplets, which are expelled when a sick person coughs, sneezes or talks. They can then be inhaled by someone standing nearby. However, these relatively heavy droplets soon fall to the ground, where they’re unlikely to infect anyone. Not so with measles.

“Measles is very distinct because it spreads through tiny droplets that sit in the air like particles of dust,” Maldonado explained. “This means that if you walk into a room two hours after an infected person was in it, you can still get the virus. This is one reason measles is one of the most infectious organisms we know.”

One person infected with measles can spread the virus to roughly 12-15 people who are not protected through vaccination or prior infection, Maldonado explained. Thankfully, in contrast to a flu shot or COVID-19 vaccine that confers only partial immunity, a two-dose measles vaccination offers almost complete protection.

“Once you’re immune, either through vaccination or prior infection, you’ve got this golden shield of protection for yourself and others,” she said.

Outbreaks are the result of under-vaccination.

Because measles spreads so easily, the measles, mumps, and rubella (MMR) vaccine plays an essential role in preventing outbreaks in the U.S. and globally, Maldonado said.

Before vaccines were introduced in the 1960s, nearly everyone contracted measles as a child – and major epidemics could cause up to 2.6 million deaths per year globally. In 2023, by contrast, just 107,000 people died from measles. The World Health Organization estimates that vaccination helped avert 60 million deaths between 2000 and 2023.

Recent outbreaks in the U.S. and other countries are almost entirely due to under-vaccination within communities, Maldonado said. A community needs at least 95% of its members to have immunity to stop measles’s spread, but the coverage rate in the U.S. has dropped to below that threshold, to about 92-93%. Globally, the vaccination rate for one dose is just 83%, and only 74% for the recommended two doses.

“The main strategy is to vaccinate young children,” Maldonado said. “This is a disease that literally could be eliminated from the face of the earth if we had enough people vaccinated with two doses.”

Decades of evidence show vaccine safety.

The measles vaccine is extremely safe and effective, Maldonado said, noting decades of studies involving hundreds of thousands of children.

Like most vaccines, the MMR shot has a small chance of side effects such as sore arm; a short-term fever or rash; or, extremely rarely, fever-induced seizure – but a consensus of experts agrees that getting the vaccine is far safer than getting measles. A large body of research consistently and unequivocally shows that the MMR vaccine does not cause autism.

One dose of the vaccine confers immunity in about 93-95% of people. Some need a second dose for the immune response to take effect, which is why two doses are now the gold standard.

Your immunity may depend on when you were born.

Adults born in the U.S. before 1957 are presumed to have had measles as a child and therefore to be immune. The first measles vaccine became publicly available in 1963, but those immunized between 1963 and 1967 may have received the less effective early version of the vaccine.

People born between the late 1960s and the late 1980s likely received just one shot of the more effective vaccine. But vaccination rates were inconsistent over the years, and a large outbreak in 1989 prompted experts to recommend two doses of the MMR vaccine. This was implemented on a wide scale and vaccine coverage improved. As a result, measles was declared eliminated in the U.S. in 2000. Between 2011 and 2019, vaccination rates remained high, with more than 95% of kindergarten-aged children being fully vaccinated for measles in 2019-20. The following year, rates began to fall, and in 2023-24, only 92.7% of kindergartners were immunized.

“Families in areas with outbreaks should be really vigilant and careful,” said Maldonado, emphasizing the importance of immunizing young children with two doses of the vaccine.

While the CDC typically recommends the first dose between ages 12-15 months, families living in or traveling to areas with outbreaks may consider having their younger babies vaccinated between 6-11 months. Maldonado emphasized that vaccinating babies during this age range is not routine, nor recommended for all children. It should only be considered with guidance from a family’s pediatrician or local public health department. While this dose will not count toward the two doses needed for lifelong immunity, it can protect them in the short term, she said.

People of all ages unsure about their vaccination history or their immunity to measles can talk with their doctor about whether a booster is recommended.