"Epidemics are four times as likely during solar maxima," said Ken Tapping, a solar physicist with the National Research Council, pointing to the striking correlation between flu pandemics and the peaks of the 11-year sunspot cycle, also known as the solar maximum.
He and two colleagues have compared flu and solar records dating back to 1729 and found a statistically significant connection. There were flu epidemics, some of them fatal to millions of people, in 1729, 1830, 1918, 1957, 1968 and 1977, years when solar activity and flares bombarded the Earth with extra radiation and cosmic rays.
There also were close correlations in other years, but they were not as pronounced. The probability of the matches happening by chance is less than 2 percent, the researchers say in a report to appear in the Canadian Journal of Infectious Diseases.
The research was done by Tapping; Dr. Rick Mathias of the University of British Columbia, an epidemiologist, and Dr. Dave Surkan, a physician. They said they are at a loss to explain the connection, and Tapping warned it could be "completely spurious." But he said the numbers are so compelling that they decided to publish the results so others can explore their findings.
"Even though things like this sound a bit strange at the start, when you look around, you find lots and lots of evidence that the sun is playing games in our environment," Tapping said.
These games get much more intense during the solar maximum, when the sun-and the sunspots that develop on its surface-release pent-up energy with the force of millions of hydrogen bombs and send clouds of radiation and searing hot gas hurtling toward Earth.
Such storms can warp Earth's outer atmosphere, set off spectacular aurora borealis displays, induce powerful magnetic currents in ground- based power lines and damage communications satellites.
The sun is also brighter at the peak of the sunspot cycle, and the amount of ultraviolet radiation hitting Earth increases, Tapping said, noting that the solar cycle is evident in tree rings and sea sediments. Tree and plankton growth is enhanced at the height of the solar cycle. There have also been suggestions that fish are more plentiful in the sea and crops grow better during that time.
Tapping, who heads the solar monitoring program at the NRC observatory near Penticton, and his colleagues offered no explanation for the connection between sun and flu in their research paper. Tapping was reluctant to speculate on how flu viruses might be affected, or mutate, as a result of increased solar activity. "We just don't know," he said.