When Wisconsin children return to school this week, close to 50,000 of them will have waivers that exempt them from vaccines, leaving them vulnerable to measles at a time when the nation has experienced its largest outbreak in 27 years.

Health officials across the U.S. have reported 1,215 cases of measles this year as of Aug. 22, the highest number since 1992, according to the Centers for Disease Control and Prevention. Measles had been declared eliminated in 2000.

“I really do think it’s purely just dumb luck that this hasn’t spread to Wisconsin,” said James Conway, a professor of pediatrics at the University of Wisconsin School of Medicine and Public Health.

Since November 2018, state health officials have investigated 382 suspected cases of measles; not one case has been confirmed.

Immunization rates of 92% to 95% are considered necessary to provide what health officials call "herd immunity." The term is used to describe a level of immunization high enough to prevent the infection from spreading to those who are susceptible, possibly triggering a widespread outbreak. The vulnerable group includes children under a year who are too young to receive vaccines and children with weakened immune systems.

Wisconsin's most recent county-by-county immunization rates paint a sobering picture. The 2018 figures measure the percentages of 5- and 6-year-olds who had received at least two doses of the measles, mumps and rubella vaccine, the amount needed to fully immunize a child.

Not a single county in 2018 came close to the 92% threshold. In fact, 40 of the 72 counties had immunization rates below 80%.

There is no comparable national statistic, but 2017 state-by-state vaccination rates for children 19 to 35 months of age showed Wisconsin trailing 36 other states, according to the CDC.

“I would not be surprised at all if I woke up tomorrow to hear that the measles outbreak had reached Wisconsin. Not surprised at all,” said Malia Jones, an assistant scientist at the University of Wisconsin-Madison Applied Population Laboratory.

“Obviously alarm bells should be going off that we’re below the threshold for herd immunity and more importantly it’s trending the wrong way,” said state Rep. Gordon Hintz, D-Oshkosh, who proposed legislation in May that would eliminate the personal exemption from immunizations.

Exemptions for personal, religious or health reasons have risen more than 9% in Wisconsin over the last three school years. In 2018-2019, exemptions totaled 49,039.

"There is no reason to think that the trend will reverse itself in the absence of any intervention such as a policy change," Jones said. "So I'd say chances are we will top 50,000 this year."

Rep. Chuck Wichgers, R-Muskego, who heads the committee that received Hintz’ bill, declined to schedule a public hearing on the matter, saying he “supports a parent’s right to decide when and how their children should receive treatment.” He declined a recent request for an interview on the issue.

“I would say that if a child was given the facts themselves and told what these diseases would be like to go through, they would choose to be given something that would not make them have to go through that disease,” said James H. Conway, a professor of pediatrics at the University of Wisconsin-Madison.

“It’s selfish for parents to put their children at risk because of an agenda about personal freedom and personal choice.”

Conway said unvaccinated children are also at risk because air travel could bring new cases of measles into the country.

Public health officials say that Wisconsin’s low immunization rate reflects a national anti-vaccine movement driven by erroneous information on social media, concerns that the shots can cause harm and by a long-ago retracted paper that suggested a false link between vaccines and autism.

The measles, mumps and rubella vaccine, often shortened to the MMR vaccine, can have side effects including fever, mild rash, temporary pain or stiffness in the joints and soreness in the arm from the shot itself.

Recent studies have indicated that about four of every 10,000 children who receive separate shots of the MMR vaccine and the chickenpox vaccine experience a very rare side effect called febrile seizures — convulsions that accompany a fever of over 100.4 degrees Fahrenheit. The condition can last for seven to 10 days immediately following vaccination.

The vaccine does not always work. About 2% to 3% of the children who receive two doses of the vaccine can still get measles, said Jones at UW.

However, a CDC web page for parents concerned about vaccine safety stresses that "getting the Measles Mumps Rubella vaccine is much safer than getting measles, mumps or rubella."

The vaccine's success may have contributed to a lack of concern about measles. Millions of Americans born after the vaccines drastically reduced measles cases have never seen a person with the illness, let alone a full-blown outbreak.

“Collective memory is low,” said Saad Omer, director of the Yale Institute for Global Health.

In the first decade after measles became a notifiable disease in 1912, the U.S. reported an annual average of 6,000 measles-related deaths. Until the arrival of the measles vaccine in 1963, an estimated 3 million to 4 million Americans were infected each year.

“One of the reasons we’re concerned, Omer said, “is because it’s a very, very unforgiving disease. It’s one of the most infectious diseases we know of.”

If a person with measles has been in a room, a child who comes in two hours later has a 90% chance of becoming infected, according to Lyn Ranta associate chief medical officer at Children’s Hospital of Wisconsin.

Once infected, it usually takes about six days before the child becomes contagious. Early symptoms are similar to a cold. Often it is not until 11 or 12 days after exposure that children begin to show the rash regarded as the hallmark of measles. That means there are often several days when parents, unaware their children even have the measles, may allow them to mix with and infect others.

About one of every 1,000 measles patients dies as a result of the infection. In some cases, hearing loss is a complication of the infection. The disease is not mild, even for children who don’t suffer complications, Conway said. “Measles is a miserable disease. It’s heartbreaking. You get a horrible headache. Everything in your body hurts.”

The implications of an outbreak extend beyond the infected.

“It’s incredibly disruptive to a community when there is a measles outbreak,” said Jones at the UW Applied Population Laboratory. “Schools could be canceled.”

In an outbreak, anyone not vaccinated against measles and anyone with a suppressed immune system would have to stay home from school. School absences would likely require parents to stay home from work to care for their children.

Each time health officials confirm a new case of measles, they would have to track down all of the people who had come into contact with them, especially those vulnerable to infection. Infected children could be quarantined for up to 21 days, said Conway.

Experts say that even a relatively small measles outbreak would probably last several months.