Swine flu continued to make headlines today when the World Health Organization today raised the pandemic threat alert to level 5 (out of 6).

Many news reports have referred to the Spanish Flu, which killed approximately 50 million people worldwide. Starting off as a relatively mild infection in the spring of 1918, the virus proved much more deadly when it returned in the fall. According to a 2008 article, this increase in lethality could have been due to a relatively weak virus mutating to become a stronger one or a respiratory bug circulating in the fall that made patients sicker in the second infection wave (J Infect Dis. 198, 1427–1434; 2008).

It seems that US and British soldiers exposed to the 1918 flu virus in the early stages of the disease were more likely to be protected from or survive the second wave of illness than those not exposed to the first wave. Given this, the authors suggested that “if a mild first [flu] wave is documented, the benefits of cross-protection during future waves should be considered before implementing public health interventions designed to limit exposure.”

What does that mean for us today? No one knows whether this outbreak will stay as is, become milder, or become more deadly. Authorities still do not know the ratio of deaths or serious illness to mild cases, meaning they don’t even know if it is currently a ‘mild’ outbreak. But lessons from the past suggest that this story will stay with us for some time.

Photo by Eneas