Many feared swine flu would kill millions. But the latest flu season proved exceptionally mild. It didn’t even cause a blip in Dutch mortality rates. - By Wim Köhler for NRC Handelsblad

Contrary to most expectations, the swine flu pandemic has claimed relatively few lives in Western countries. In fact, the Dutch influenza mortality rate of the past flu-season was the lowest it has been in ten years. Official statistics show the swine flu pandemic killed 60 people in the Netherlands. In the mortality figures the national statistics office Statistics Netherlands (CBS) posts on its website every week, the flu does not appear at all.

"In this last winter we didn’t see any flu-related spikes in mortality," said Carel Harmsen, a statistical analyst for CBS. The CBS analyses the Dutch mortality rate by the week. A high rate is usually the result of extreme temperatures and the flu.

Roel Coutinho, managing director of the Dutch Centre for Infectious Disease Control, said he concurred with the CBS’ analysis. "We are at the end of an incredibly mild flu season," he said. "I do believe some more than the 60 people we officially counted died from it, but so few that they don’t even show in the CBS statistics."

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Not even a blip
The latest flu season, which began shortly after the discovery of a new influenza virus in Mexico and the US a year ago, has come to a surprisingly gentle end. In June, the World Health Organisation officially declared it a pandemic. Experts warned that hundreds of thousands, perhaps millions of people worldwide might succumb to the newly discovered virus.

In the Netherlands, the average flu season claims some 700 lives. If the winter flu strikes hard, especially amongst the elderly, an additional 1,500 people usually die. Last year, a mild wave of swine flu reached the Netherlands in October and November, but the seasonal flu that was expected afterwards failed to materialise. The same happened in the rest of Western Europe and in North America.

"Mortality spiked in the winter, but that can be entirely attributed to it being the coldest winter in a decade," Harmsen said. "During a cold spell, every degree the temperature drops causes an additional 15 to 20 deaths per week."

As opposed to the ‘normal’ flu, swine flu killed younger rather than older people. Experts had predicted the young would catch it first, because they travel more and have more social contacts, and would soon infect the elderly. The Dutch government bought 34 million doses of vaccine against the virus and offered these to children between 6 months and 5 years, pregnant women, people with special health risks and everyone over 60.

"The big surprise was that people born before 1957 were relatively well protected against the new pandemic flu virus," Coutinho said.

Old people immune
Molecular research into the virus’ origin has cast light on the immunity older people seem to enjoy. Swine flu has traces of the H1N1 flu, itself an offspring of the Spanish flu, that was common until 1957. Many people born before that year had therefore been exposed to a similar virus and developed some resistance to swine flu.

When swine flu had all but disappeared by December, there was still plenty of time left for traditional seasonal flu, caused by H1N1, H3N2 or B viruses, to strike. Amazingly though, nothing happened.

The question remains why. "Not a clue," Coutinho said. "You should not forget this is the first time in history we are able to follow a new influenza virus at the molecular level as it spread across the globe." According to Coutinho, the viruses normally associated with seasonal flu have been virtually eliminated by the now omnipresent H1N1 swine flu.

Possible explanations
It is not the first time viruses have been displaced by successors. Pandemics that spread new viruses often deliver the final blow to earlier strains, because people’s immune systems have grown accustomed to those over the years .

Still, this theory does not explain the absence of traditional seasonal flu in Western Europe earlier this year. Even if all other influenza viruses have been displaced, why didn’t the swine flu strike again during the cold spell that made the Netherlands extra susceptible to it?

"One can only speculate as to why," Coutinho said. He cited an article published in the British medical journal The Lancet that claimed that only half of all the people infected in England fell ill. This raises the possibility that many people became infected without knowing it and quickly developed resistance to the virus rather than suffer from it.

Meanwhile, French experts published a competing theory in the Eurosurveillance journal. They argued that other viruses in the respiratory track could have prevented the new influenza virus from spreading. The team compared French outbreaks of influenza, RS-virus and cold-causing rhinoviruses and found that they usually follow in perfect succession. The team theorised that, at any given time, only a single virus could maintain dominance over French airways. "An interesting theory," according to Coutinho. "But for now, only a theory." Evidence in the Netherlands suggests it may be correct though. Many Dutch people had particularly long lasting colds in January and February.