NEW YORK, US (AP) — As the US wrestles with its biggest whooping cough outbreak in decades, researchers appear to have zeroed in on the main reason: The safer vaccine that has been in use since the 1990s loses effectiveness much faster than previously thought.

A study published in yesterday's New England Journal of Medicine found that the protective effect weakens dramatically soon after a youngster gets the last of the five recommended shots around age 6.

The protection rate falls from about 95 per cent to 71 per cent within five years, said researchers at the Kaiser Permanente Vaccine Research Centre in Oakland, California.

The US has had more than 26,000 whooping cough cases so far this year, including more than 10,000 in children ages seven to 10.

"The substantial majority of the cases are explained by this waning immunity," said Dr William Schaffner, an infectious-disease specialist at Vanderbilt University.

In light of the findings and earlier, similar research, health officials are considering recommending another booster shot for children, strengthening the vaccine or devising a brand new one.

But "there's nothing in the pipeline that's close", said Dr Tom Clark of the Centers for Disease Control and Prevention.

Whooping cough, or pertussis, is a highly contagious bacterial disease that can strike people of any age but is most dangerous to children. Its name comes from the sound youngsters make as they gasp for breath.

It used to be common, causing hundreds of thousands of illnesses annually and thousands of deaths. Cases dropped after a vaccine was introduced in the 1940s, and for decades, fewer than 5,000 a year were reported in the US.

Because of side effects that included pain and swelling at the injection site, fever and apparently, in rare cases, brain damage, the vaccine was replaced in the 1990s. The newer version used only parts of the bacterium instead of the whole thing and carried fewer complications.

But cases of whooping cough began to climb, sometimes topping 25,000 a year during the past decade. Also disturbing: The proportion of cases involving children ages seven to 10 — most of them vaccinated — rose from less than 10 per cent before 2006 to nearly 40 per cent this year, according to the CDC.