The world must remain on its guard against H1N1 influenza, which has been mild so far but could become more serious as the northern hemisphere heads into winter, the head of the World Health Organization said on Sunday.

Margaret Chan, on a visit to Tanzania, noted that most people infected with swine flu had suffered only mild symptoms but it affected certain groups such as pregnant women and people with underlying medical conditions much more severely.

RELATED: H1N1 Survival Guide

H1N1 Live Shots

Complete Coverage

"Looking ahead in the weeks and months ahead, especially for countries in the northern hemisphere, when they will be going into the winter, we need to maintain our vigilance and see how the disease will evolve," Chan told reporters.

The Geneva-based United Nations agency declared H1N1 a full pandemic in June. It has now spread to nearly 180 countries.

In its latest update last week, the WHO said laboratory-confirmed deaths from the flu strain totalled at least 1,462. While the WHO says at least 177,000 people have caught the disease the real number is probably in the millions.

Chan noted that many people in Africa already suffer from diseases such as HIV, tuberculosis and malaria, or are undernourished, putting them in the vulnerable category.

"These people would be at greater risk of getting severe disease," she said.

Chan repeated the WHO's promise to ensure that developing countries receive supplies of vaccines against H1N1 when they are available, recalling that France's Sanofi-Aventis and Britain's GlaxoSmithKline had pledged to donate 150 million and 50 million doses respectively.

But with the northern hemisphere heading into winter, the priority would be developing countries there, she said.

She was talking on her arrival in Tanzania for a visit focusing on efforts to prevent malaria.