A case of whooping cough in an unvaccinated child has been confirmed at Sycamore Elementary School in the Vail Unified School District.

Parents of students at the school have received a letter saying their children may have been exposed to whooping cough.

The letter warns that the infection can cause serious complications, especially for pregnant women, infants and people with weakened immune systems. It also notes that even children who have been vaccinated can contract whooping cough.

The Sycamore case is far from isolated. Fourteen cases have been confirmed in the district since Aug. 1, health department officials say.

The Pima County Health Department last weekend declared an outbreak of whooping cough at Empire High School, also in the Vail District.

In the Empire High School case, students who have not been fully vaccinated against whooping cough, which is also called pertussis, are not allowed to attend classes or school-related activities for three weeks or longer, depending on whether future cases are identified.

Three students who had been vaccinated were infected in the Empire High School case, and it’s the only school that Health Department officials have labeled with outbreak status.

Pima County Health Department Director Dr. Francisco Garcia said Friday that three of the 14 confirmed cases since Aug. 1 have been in children who were not vaccinated.

It’s a good time for parents to make sure their children are up-to-date on immunizations, he said.

As of Oct. 19, 69 whooping cough cases had been reported in Pima County, already higher than the total of 46 for all of last year.

Nationally, 48,277 cases of whooping cough were recorded last year, which is the highest number reported in the U.S. since 1955.

Health officials attribute the spike to a combination of factors. Fewer parents are vaccinating their children, and more are starting but not completing the series of vaccinations. But that’s not the only reason.

The vaccine is effective only about 70 percent of the time. And a switch from what’s called a “whole cell” pertussis vaccine to a more purified “acellular” version appears to be causing the childhood pertussis vaccine to wear off faster than in the past.

Many adults don’t realize that they need a “booster” called Tdap — tetanus, diptheria and pertussis — to fully protect themselves. And while adults may not get terribly ill from whooping cough, they could pass it along to a baby who is too young to be vaccinated. Babies die of whooping cough at a rate of about one or two per year in Arizona.