UPDATE 1-Swiss bought H1N1 flu vaccines from Novartis, Glaxo

*Switzerland bought 13 mln doses of vaccine

*Doses to be delivered in several tranches from end of Sept. (Adds details, background)

ZURICH, Sept 18 (Reuters) - Switzerland has bought 13 million doses of the H1N1 swine flu vaccine from drugmakers Novartis AG (NOVN.VX) and GlaxoSmithKline (GSK.L), the Swiss government said on Friday.

The doses will be delivered in several tranches from the end of September, the government said. It did not say how much it was paying the companies for the vaccines.

Health workers and those believed to be more at risk from the virus, such as pregnant women and people with chronic heart or pulmonary disease, should get the vaccine first, the government said.

Switzerland had 7.7 million inhabitants at the end of 2008, according to the Federal Statistical Office.

Because it is a new strain, infectious disease experts had initially anticipated that people would likely need two doses of any H1N1 vaccine to get full immunity against swine flu.

But evidence is mounting that tight supplies can be stretched by not using two doses. Earlier this week, Glaxo said a single shot of the vaccine protected people from the virus. [ID:nLE685651]

The World Health Organisation warned on Friday that annual production of the vaccine would be "substantially less" than the 4.9 billion doses previously forecast and that current supplies were "inadequate for a world population in which virtually everyone is susceptible to infection".[ID:nLI179011]

The H1N1 strain of flu, declared a pandemic in June, could eventually infect one third of the world's population, or 2 billion people, according to the WHO.

Japanese media reported earlier this month that Japan's health ministry was set to buy enough H1N1 flu vaccine for nearly 50 million people from Novartis and Glaxo, while Singapore has already bought one million doses of the vaccine from Glaxo. (Reporting by Katie Reid; Editing by Rupert Winchester)