Measles virus stays airborne for two hours
For decades, the ability to remain infectious in a room long after a sick person has left was a trait understood only in the context of airborne pathogens like tuberculosis. Measles, however, does something that even influenza cannot match. The measles virus stays alive and contagious in the air for up to two hours after the infected person has walked away. This is not a feature of a sophisticated organism — it is a physical property of the virus itself that allows it to remain airborne for up to two hours. A child with measles can enter a classroom, cough once, leave, and two hours later a child who walks into that same empty room can inhale enough virus to become infected. No other common childhood disease behaves this way. The virus does not need direct contact. It does not need a sneeze in the face. It needs only air that was breathed by someone who was sick.
This staying power is why the current outbreak in the United States has become so difficult to stop. In South Carolina, the state Department of Public Health confirmed in October 2025 that an outbreak had hit the upper region of the state with eight known cases. Five of those individuals caught the virus within the past month. The department defined an outbreak as three or more cases linked to one another. All those involved were unvaccinated and had no immunity. The state epidemiologist, Dr. Linda Bell, said that officials had not yet identified where two individuals contracted the virus. She anticipated more cases would be identified. The virus was moving through communities where people had no protection.
The scale of the problem is now visible in the national numbers. Already this year, health officials have recorded 2,295 cases, according to a July 21 report from the Johns Hopkins Bloomberg School of Public Health. That surpasses the 2,286 cases documented in all of 2025. Before 2025, the last time the United States recorded more than 2,000 cases was in 1992. The U.S. Centers for Disease Control and Prevention will report similar numbers when it updates its case count this Friday. An explosive outbreak in South Carolina and a simmering one in Utah have driven much of the increase. Another 40 states, as well as New York City and Washington, D.C., have also reported cases. The virus is not confined to one region. It is everywhere.
The Forest Fire Metaphor for Measles Transmission
William Moss, an infectious disease epidemiologist at the Johns Hopkins Bloomberg School of Public Health, uses a forest fire metaphor to describe what is happening. He says the outbreak that began in West Texas in January 2025 sent sparks out across the country. Some of those sparks landed in vulnerable communities with low vaccination rates, and large outbreaks followed. Other sparks landed in communities like Maryland, where Moss lives. Maryland has had a number of importations over the past two years but no sustained outbreak. The difference is not luck. It is vaccination coverage. In Maryland, the virus lands in a community with fairly high vaccine coverage, and the fire dies out.
The threshold for stopping measles is brutally precise. The virus is so contagious that at least 95 percent of people in a community must be vaccinated to prevent it from spreading. This is not a recommendation. It is a mathematical requirement based on how many people each infected person can infect. Across 33 states, the average two-dose MMR vaccination rate fell from about 94 percent before the COVID-19 pandemic to about 91 percent afterward, researchers reported in JAMA in 2025. That drop of three percentage points is small in absolute terms. But it is enough to push many communities below the 95 percent threshold. The virus found the gaps.
The age distribution of cases confirms that this is not a sudden problem. Only about 20 percent of cases are in children younger than five. About 50 percent of cases are in children and teenagers aged five to 19. About 30 percent are in adults older than 20. This wide age range is an indicator that under-vaccination has been building for years. If the problem were recent, the youngest children would be hit hardest. Instead, the virus is finding unvaccinated people of all ages. The gaps in protection have been accumulating for a long time.
The Fragile Status of Measles Elimination in North America
The United States has held measles elimination status since 2000. That designation means that the virus is not constantly circulating within the country. Imported cases still arrive, but they do not lead to sustained transmission. That status is now in serious jeopardy. Moss says he would be shocked if the United States does not lose its elimination status this year. The loss of that status means that there has been endemic measles virus transmission for 12 months or more. It is an arbitrary threshold, but it exists to allow countries to maintain elimination status if they have importation events followed by short-term transmission that dies out. The United States may no longer meet that standard.
The CDC and other agencies are doing genetic analyses on the measles virus to see if there are major genetic differences between cases. If the genetics changed significantly over time, that would be evidence of repeated importations rather than continuous transmission. Moss does not think that is what they will find. The virus appears to be circulating continuously.
This is not just a U.S. problem. The ongoing outbreak has really been a North American outbreak. Mexico has had over 12,000 cases. Canada lost its elimination status in November 2025. There has been a lot of movement of the virus across borders between those two countries and the United States. When Canada lost its elimination status, the whole World Health Organization region of the Americas lost it. Elimination is a very fragile state. It is very hard to maintain because of the threat of importation from other countries. Beyond the Americas, the really big, devastating outbreak this year has been in Bangladesh, with close to 50,000 confirmed cases. Measles cases are often way underreported, so the true numbers are likely even higher.
The Global Impact of Withdrawn U.S. Health Support
The big development over the past year and a half has been the withdrawal of the United States from supporting many global public health efforts, including immunization. The United States withdrew from the World Health Organization. It withdrew huge amounts of funding. It withdrew support for Gavi, the Vaccine Alliance, which supports many low-income countries not only in routine immunization for measles but also in mass vaccination campaigns. The dismantling of USAID, which supported many immunization programs, has also hurt. The United States is not alone in this. European countries have also withdrawn support for global public health initiatives.
Moss says there are holes in the protective immunity of the U.S. population, and these holes are often not visible until measles virus starts circulating. The virus finds them. The steady increase in nonmedical exemptions has made the problem worse. Many states have tried to relax their school entry requirements through legislation, but a lot of that legislation has not passed. In general, parents in the United States recognize the value of vaccinations. There has been a few percentage-point decline in measles vaccine coverage among kindergartners, but it is still quite high. Moss does not think there will be a dramatic drop. Most parents are in favor of vaccination.
Dr. Jon LaPook reported for CBS News that a measles outbreak centered in Texas has driven U.S. cases to the highest number in 33 years. Measles is preventable with a vaccine, but because so many are not getting it, doctors are facing a new challenge. They are treating a disease that should not exist in this country. The United States has five months left in 2026. By the end of the year, it may no longer be able to call itself a country that has eliminated measles. The virus is winning because the air carries it, the gaps in vaccination are wide enough, and the global support systems that once helped contain it have been dismantled.