WASHINGTON (Reuters) – The new pandemic of H1N1 swine flu is causing a strong second wave of disease in many Northern Hemisphere countries, according to the World Health Organization.

While the United States, China and Australia have begun vaccination, other countries have not and it is unlikely many people will be protected from the virus before November. Here are some possible ways the pandemic could play out:

BECOMING PART OF THE MIX

The new H1N1 virus is a distant cousin of an H1N1 strain that has been part of the seasonal influenza mix for decades. Early surveillance suggests the new swine flu strain may supplant seasonal H1N1 and become part of the common circulating viruses. This could be good news as the seasonal H1N1 had developed resistance to the antiviral drug oseltamivir, Roche AG and Gilead Science’s pill sold under the Tamiflu brand name. But most viruses eventually mutate and health experts would not be surprised to see swine flu acquire resistance. That is why companies are working to develop newer and better influenza drugs.

The U.N’s senior technical expert on influenza, Dr. Julie Hall, said it takes two to three years for a new influenza virus to infect enough of a population to create broad immunity.

ADDING A NEW DIMENSION

Because flu viruses mutate and recombine to form new strains, people remain vulnerable to flu all their lives. This is why the vaccine must be reformulated each year. H1N1 has been remarkably stable since it began infecting people widely in March and April this year. But experts predict once it has infected a certain proportion of the population — no one knows exactly what proportion — it will start to change.

Right now the H1N1 vaccine is a good match against the virus, and most adults and older children will get good protection with a single dose. If the virus “drifts,” the vaccine will have to be reformulated to match, just as with the seasonal flu vaccine. The process takes about six months.

Health officials will monitor closely for this to happen. In past pandemics, notably the one in 1918 that killed between 40 million and 100 million people globally, a first wave of relatively mild influenza was followed by a second wave of severe disease months later.

WHEN PIGS FLY

It is also possible that the virus will recombine, swapping genetic material with other flu viruses, such as the seasonal H3N2 virus. A special concern is that someone could become infected with both the H1N1 virus and the H5N1 avian flu virus. They could then combine to create an especially virulent new virus “that would have very unpleasant consequences for humanity,” United Nations special pandemic coordinator Dr. David Nabarro told a World Bank briefing on Sunday.

Bird flu is still circulating and has infected 442 people, killing 262 of them, since 2003, the World Health Organization says. It is difficult for people to catch bird flu but when they do it is highly deadly. If a new virus had H1N1′s infectivity and H5N1‘s deadliness, it could be devastating.

Many companies are working on H5N1 vaccines, which could give the world a head start on a new vaccine if any eventual new mutant closely matches the strain being used to make it.

LACKING VACCINES

Whatever the virus does, the world lacks the capacity to vaccinate most of the population against flu. The WHO estimates worldwide production capacity for pandemic vaccines at approximately 3 billion doses a year, which would be enough to cover fewer than half the world’s 6.8 billion people. The WHO is pressing rich countries to buy and donate vaccine to poorer countries.

Hall told the World Bank gathering that the first wave of the swine flu pandemic affected wealthier nations like the United States, Australia and Japan, where it is still active.

“But what we are seeing now is that the virus is beginning to penetrate into some of the poorest communities in the world,” she said. There it may cause “explosive outbreaks” among young and working-age adults — a particular problem for countries with younger populations.

Joy Phumaphi of the World Bank estimates that even a mild epidemic will reduce world gross domestic product or GDP by 0.7 percent. A more severe epidemic could reduce GDP by 3 percent, as it not only takes people out of the workforce for days or weeks, but requires them to seek expensive medical care.

(Editing by Chris Wilson)