UPDATE 1-WHO says health workers priority for H1N1 vaccine

* Children, pregnant women might also go to front of line

* All countries in world will need H1N1 swine flu vaccine

* Vaccine yields so far "poor", other strains being studied

* WHO reviewing global vaccine capacity forecasts

(Adds further details, background)

By Laura MacInnis and Ben Hirschler

GENEVA/LONDON, July 13 (Reuters) - Healthcare workers should get priority access to H1N1 flu vaccinations to ensure health systems keep functioning as the swine flu pandemic spreads around the globe, the World Health Organisation said on Monday.

Marie-Paule Kieny, WHO director of the Initiative for Vaccine Research, said the agency's experts had concluded every country in the world would need access to vaccines, which drug companies are now racing to produce.

Disappointingly, Kieny said, yields from growing the new virus in chicken eggs -- the mainstay of flu vaccine production -- had so far been "poor", at only 25 to 50 percent of that achieved with normal seasonal flu strains.

Alternative strains are now being studied that will hopefully increase output, she added.

The WHO raised the global flu alert to the highest level on June 11, declaring a pandemic is underway from the H1N1 virus strain commonly known as swine flu.

The new disease has so far caused mild symptoms in most patients, though at least 429 have died and experts fear the death toll will climb steeply without a widespread immunisation programme.

Pregnant women have been vulnerable to serious effects and Kieny also confirmed obesity as a risk factor for severe complications.

"The (WHO group of experts) recommended first that healthcare workers should be immunised in all countries in order to retain a functional health system as the virus evolves," she told reporters in a conference call.

"In terms of the other groups it depends on the strategy a country wants to follow."