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LONDON, Feb 19 (Reuters) - GlaxoSmithKline Plc <GSK.L> cut the prices on its range of HIV drugs offered to developing countries, marking the fifth such discount since 1997.

The most significant reduction is an almost 40 percent cut on Ziagen, a pill the World Health Organisation recommends as a first- and second-line treatment particularly for children. The average discount across its 14 not-for-profit HIV medicines was 21 percent, the company said on Tuesday.

More than 33 million people globally are infected with HIV, which is incurable and deadly. There is no vaccine and drugs that help control the infection do not stop its spread and are not available to most people.

"These prices will take effect immediately for public sector customers and not-for-profit organisations in the least developed countries and sub-Saharan Africa (as well as other eligible countries)," the company said in a statement.

The decision comes after Thailand stunned drug makers in late 2006 when it overrode the patent on Merck's AIDS drug Efavirenz, arguing it could not afford patented drugs for its national healthcare scheme.

Months later it did the same on a Sanofi-Aventis <SASY.PA> heart medicine and an AIDS drug made by Abbott Laboratories <ABT.N>, which refused to register several new medicines in Thailand.

Drugmakers themselves say they are doing more than ever to help the world's poor with a raft of initiatives designed to get healthcare to millions who cannot afford to pay Western prices. (Reporting by Michael Kahn; Editing by David Holmes)

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