WEDNESDAY, Feb. 6 (HealthDay News) -- The bacteria responsible for whooping cough may be evolving into different strains, and the current vaccine can't offer complete protection against these new strains, researchers report.

In recent years, cases of whooping cough have risen dramatically. Tens of thousands have been sickened, and 18 deaths have been reported, mostly in infants, according to the U.S. Centers for Disease Control and Prevention.

Several recent studies have focused on the potential of waning immunity from the whooping cough vaccine over time. In a letter in the Feb. 7 issue of the New England Journal of Medicine, researchers pointed out that there may be another culprit: an evolution in the bug itself.

The vaccine contains a number of components that help give the body immunity against the whooping cough bacteria (Bordetella pertussis). One of these components is called pertactin.

Strains of the whooping cough bacteria that are pertactin-negative have been found in Japan, France, Finland and, now, the United States, the researchers reported in the letter. That means that at least one component of the current vaccine is ineffective against these newly found strains.

"There are several theories as to why we're seeing more pertussis," said Dr. Kenneth Bromberg, chairman of pediatrics and director of the Vaccine Research Center at the Brooklyn Hospital Center, in New York City. "One theory is better diagnosis. A second [theory] is that the vaccines are not as good in terms of longevity as the whole-cell vaccine was. The third theory is that there have been genetic changes in the strains of pertussis, such that it makes new strains immune or relatively immune to the current vaccines."

"The thought is that the bacteria have gotten smart and are eliminating the pertactin in themselves," said Bromberg, who was not involved in the current research.

A second letter in the same journal issue addressed another of the theories that Bromberg outlined: the effectiveness of the current vaccine.

In the late 1990s, the vaccine was switched from a whole-cell vaccine to what's known as an acellular vaccine.

"The whole-cell vaccine contains the whole cell of killed bacteria, so every protein that the bacteria makes is in it, forcing the person to mount an immune response to every part of the bacteria," said the letter's co-author, Dr. Paul Cieslak, medical director of the Oregon Immunization Program, in Portland. "With the new, acellular vaccine, they picked the proteins they thought were most important. Without the whole cell, there are fewer side effects, but it may be a little less effective."

Oregon experienced a significant increase in the number of whooping cough cases last year. Oregon maintains statewide records on immunizations, as well as records of who has had a confirmed case of whooping cough. This information allowed Cieslak and his colleagues to go back and review what type of vaccine had been given to those who later were infected with whooping cough.

They found that the reported cases of whooping cough were significantly lower in youngsters who had their first immunization with the whole-cell vaccine rather than the acellular vaccine.

Does that mean that whole-cell vaccine should make a comeback? "No one is making the argument for whole-cell vaccine," Cieslak said. "Without the whole cell in the vaccine, there are fewer side effects. It may be a little less effective, but it's got to be safe."

Bromberg agreed. "The whole-cell vaccine isn't coming back. The acellular vaccine is significantly better in terms of not causing reactions, but it doesn't work for as long as we thought," he said.

Cieslak said that even though the current vaccine isn't perfect, "we certainly recommend getting the vaccine. You are still far better off with the vaccine."

Bromberg added that experts are trying to figure out ways to better use the current vaccine. For example, the CDC recently added a recommendation that all pregnant women should receive the whooping cough vaccine late in pregnancy to offer protection to their newborns.

More information

Learn more about preventing whooping cough from the U.S. Centers for Disease Control and Prevention.