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ATLANTA - Health officials on Monday celebrated a faster treatment for people who have tuberculosis but aren't infectious, after investigators found a new combination of pills knocks out the disease in three months instead of nine.

That means more people are likely to finish their treatment for latent tuberculosis, Centers for Disease Control and Prevention officials said.

"New, simpler ways to prevent TB disease are urgently needed, and this breakthrough represents one of the biggest developments in TB treatment in decades," CDC director Dr. Tom Frieden said, in a prepared statement.

For decades, people infected with TB bacteria but not ill have been treated with a special TB pill, isoniazid, taken once a day for nine months. It's been the standard regimen despite problems getting people to take the pill every day.

But in one of the largest federal trials to examine preventive tuberculosis therapy, investigators found that another regimen was just as effective. Just once a week and for just three months, patients took a larger dose of isoniazid and also a dose of another antibiotic, rifapentine.

About 82 per cent of the people in the three-month regimen completed the full treatment, while just 69 per cent on the nine-month regimen did. Rates of the most serious side effects were the same for both regimens.

What's more, only seven cases of TB disease developed in people on the new treatment, compared with 15 in the standard group.

"It was quite effective," said Dr. Wafaa El-Sadr, a Columbia University professor of medicine and epidemiology who was involved in the study.

The three-month regimen is also more expensive. The medicines alone cost about $160, most of that from the price of rifapentine. Nine months of isoniazid costs less than $6.

The costs of both regimens grow when lab tests and other aspects of care are thrown in, but the three-month regimen still ends up being more than twice as expensive as the standard treatment.

The study was led by Dr. Timothy Sterling of Vanderbilt University and was presented Monday at the American Thoracic Society International Conference in Denver.

The CDC is working with consultants to examine the study's results and draft new guidelines for treatment of latent TB. The guidelines should be finished later this year, agency officials said.

Tuberculosis is caused by bacteria spread from person to person through the air. TB usually affects the lungs, and can lead to symptoms such as chest pain and coughing up blood. Globally, it kills about 1.7 million people each year.

Thanks to antibiotics and other measures, the TB rate in the United States has been falling for years. Last year, it hit an all-time low -- a total of 11,181 reported cases of TB illness.

But more than 11 million Americans have latent TB, meaning they are infected with the TB bacteria but have not had symptoms and are not infectious.

About five per cent to 10 per cent of people with latent TB develop the disease if not treated, meaning they are a major obstacle to eliminating TB in the United States.

"The 11 million persons with latent TB represent a ticking bomb. They're the source of future TB cases," said Dr. Kenneth Castro, director of the CDC's Division of Tuberculosis Elimination.

Most Americans with latent TB don't know they are infected, but testing has been targeted at groups of people who tend to have higher rates of TB infection or who are more susceptible to TB infection progressing into illness. About 300,000 to 400,000 Americans with latent TB start treatment each year.

But many don't stick with it, sometimes because they feel well and don't see the need to keep taking a pill against an illness they haven't developed. Some don't like that they cannot drink alcohol while taking isoniazid.

The study looked at about 8,000 people with latent TB in the United States, Canada, Brazil and Spain. They were followed for nearly three years from the time they started the study. Most of the top TB research centres in the United States were involved.

About half were given the standard treatment, a daily 300 milligrams dose of isoniazid for nine months, and they took it on their own. The other half were put on a 900-milligram dose of isoniazid and a 900-milligram dose of rifapentine, but did it in front of a doctor or other health-care worker.

The researchers acknowledged that follow-up studies are needed to see if patients on the three-month regimen are as faithful at taking their medicine when they aren't being monitored.

Also, it's not clear how well the strategy would work in countries where TB is more common and the odds of re-infection are much higher, health officials said.