By Paul Simao

ATLANTA (Reuters) - The number of infants dying from whooping cough, once a major killer of children in the United States, is rising despite record high vaccination levels in the nation, federal health officials said on Thursday.

Researchers with the Centers for Disease Control and Prevention speculated that the worrying trend might be an indication that the bacteria that causes whooping cough was becoming more common in the nation.

Whooping cough, also known as pertussis, is marked by spasms of coughing followed by vomiting and a "whoop" as suffers can finally suck in air. It occurs in all age groups, but is especially dangerous in newborns who have not yet developed strong immune systems.

In 2000, the latest year that data was available, there were 7,867 reported cases of the disease and 17 fatalities, compared to 7,297 cases and 14 deaths the previous year.

Infant whooping cough deaths rose steadily in the 1980s and 1990s.

Dr. Kris Bisgard, an epidemiologist who works in the CDC's national immunization program, said it was important for parents to get their infants vaccinated against whooping cough and to keep them away from anyone suffering a severe cough.

Bisgard, however, dismissed suggestions that a recent shortage of the diphtheria, tetanus and pertussis vaccine, known as DTaP, might be partly responsible for the rising number of cases.

Supplies of the childhood vaccine had been limited in the past 18 months due to production glitches at manufacturing plants. Earlier this month, the CDC said the shortage had ended, allowing for the resumption of routine vaccinations.

"We don't think that the shortage had any impact on circulation" of the bacteria, said Bisgard, who noted that all the deaths in 2000 occurred among infants under the age of 4 months, too young to have completed the first three of five recommended DTaP vaccinations.

Health experts advise that infants receive three shots for whooping cough at 2 month intervals after birth, followed by a fourth dose about a year later and a booster between the ages of 4 to 6 years.

Whooping cough vaccination rates for children between the ages of 19 and 35 months in the United States have been found to be above 90 percent, according to the CDC.

The CDC said it was beginning studies to identify the risk factors for severe and fatal pertussis and better understand the disease. In 2000, nearly half the deaths inexplicably occurred among Hispanic infants.