Reported cases of whooping cough in Maine more than tripled this year, in line with a national trend that resulted in the worst year of pertussis infection in the United States in more than 50 years.

Maine’s increase from 202 cases in 2011 to 701 cases so far this year has state health officials encouraging most Mainers to get vaccinated if they’re unclear about their immunization history.

“If you’re not sure, just go ahead and get it,” said Dr. Stephen Sears, state epidemiologist.

The national increase — from 18,719 cases in 2011 to 40,411 cases through the end of last week — is the highest number since 40,000 cases were reported in 1959, according to the U.S. Centers for Disease Control and Prevention.

The increase is rooted in a variety of factors, Sears said, including the fact that the vaccine was made purer in the 1990s to reduce side effects. Now, to be effective, the vaccine requires stricter adherence to the five-shot immunization series through infancy and childhood, and booster shots in adulthood.

“Some people are under-vaccinated because they missed a dose along the way,” Sears said.

Other people are anti-vaccination because they dispute the safety of or need for immunization, he said. Still others don’t like getting shots or don’t know they should be getting booster shots as adults.

“It’s really the right combination of factors that’s causing this increase,” Sears said.

In Maine, more than 90 percent of children receive vaccines, but the immunization rate for pertussis drops to 70 percent among children ages 11 to 19, state health officials said. Only about 3 percent of Mainers actually refuse to be vaccinated, Sears said.

“The risk of getting sick with the vaccine is very small,” he said. “The risk of getting sick without it is great.”

Federal officials attribute some of the increase to better detection and reporting.

Significant pertussis outbreaks have been reported this year in counties across Maine, including Cumberland (219 cases), Somerset (99), York (69), Penobscot (67) and Androscoggin (59), according to the Maine Center for Disease Control and Prevention.

Maine health officials reported a total of 53 pertussis cases in 2010.

The state doesn’t provide town-by-town statistics to protect patients’ identities in small towns, said John Martins, spokesman for the Maine Department of Health and Human Services.

Other states reporting significant increases in pertussis cases this year include Vermont, Minnesota, Wisconsin, Colorado and Washington, according to the federal CDC.

Pertussis is a bacterial respiratory infection characterized by persistant coughing fits that are marked by high-pitched whooping sounds when a person inhales.

Before pertussis immunization became widespread in the 1940s, most children developed whooping cough; 150,000 to 260,000 cases were reported annually, causing as many as 9,000 deaths each year, according to the CDC.

Today, deaths from whooping cough are relatively rare and occur mostly among infants.

From 2000 to 2008, 181 people died from pertussis; 166 or 92 percent were less than 6 months old, according to the CDC.

No pertussis deaths have been reported in Maine since 2006, when a pediatric patient died from the disease in Cumberland County, Martins said.

The CDC reported 16 pertussis-related deaths in the United States this year through Dec. 8; ages weren’t noted. Among pertussis patients under age 1, more than half are hospitalized; among those infants, 1 in 5 get pneumonia and 1 in 100 die.

Studies show that mothers are often responsible for infecting their babies. To further protect infants, the CDC stiffened its immunization guidelines in October, recommending that all pregnant women should be vaccinated 27 weeks into each pregnancy, regardless of their immunization history.

“All infants, pregnant women and anyone around them should be immunized,” Sears said.

Pertussis vaccinations, which also immunize against diphtheria and tetanus, are readily available through primary care practices and health clinics.

Ask a doctor about health conditions that may preclude immunization.

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