Pregnant women and other people at high risk should be vaccinated against the H1N1 swine fluvirus as the cold weather begins to bite in the northern hemisphere, the World Health Organization (WHO) said on Tuesday.

It voiced concern that some vulnerable people are shying away from the pandemic vaccine, which the WHO stressed had not caused any unusual side effects in hundreds of thousands of people to have received it worldwide so far.

Certainly the fact that the vaccine isn't being used by those who would have access to it and who would be in priority risk groups is of concern, yes, WHO spokesman Gregory Hartl told a news briefing.

At least 5,712 people have died from the swine flu virus, according to the United Nations agency. Health workers, pregnant women and people with conditions such as asthma are deemed at greatest risk and should be top priority for getting the shot.

We have seen many, many instances of people in high risk groups such as pregnant women who have very severe disease or outcomes. These outcomes could be in all likelihood avoided if one were to get vaccinated, Hartl said.

Unlike seasonal flu, which is most dangerous to the elderly, H1N1 is hitting younger adults and children especially hard.

The WHO recommends a single vaccine dose for protecting adults againstH1N1. It has also urged governments to consider giving a single dose to as many children under age 10 as possible, but says more research is needed into child dosages.

In the United States, studies show that children under the age of 9 will need two doses of the vaccine to be fully protected, officials there said on Monday [ID:nN02462530].

Parts of North America and Europe have already crossed the epidemic threshold, ahead of the normal influenza season which peaks in January-February for the northern hemisphere, WHO says.

There is substantial influenza activity and we would expect to see more. Certainly the indications are that this will become something quite widespread across the northern hemisphere temperate zones as we go forward through the late autumn and winter, Hartl said.

But the WHO is reluctant to speak of a second wave of outbreaks, as the virus may have just been less active in the summer months, he said. Colder weather favors its spread.

Countries from China to the United States have rolled out vaccination campaigns and the rare side effects have been mild, in line with those caused by seasonal flu vaccines, Hartl said.

GlaxoSmithKline and Sanofi-Aventis are among some 25 drugmakers companies producing pandemic vaccine.

Medicine Winter