Pandemic not always preceded by spring flu

Existing data are insufficient to conclude the 1918-19 Spanish flu pandemic was preceded by a mild, so-called spring wave, U.S. researchers said.

Scientists at the National Institute of Allergy and Infectious Diseases said their analysis of 14 global or regional influenza epidemics in the past 500 years reveals no consistent pattern of wave-like surges of disease before major outbreaks.

A commonly held belief is that severe influenza pandemics are preceded by a milder wave of illness in the spring. Some accounts suggest the 1918-19 flu pandemic followed that pattern, researchers said.

But the evidence does not bear that out, they said.

Dr. David M. Morens and Dr. Jeffery K. Taubenberger said two other flu pandemics of the 20th century, those of 1957 and 1968, generally showed a single seasonal occurrence.

Past flu pandemics make it difficult to predict the future course of current H1N1 virus, which first emerged last spring, the researchers said.

Efforts to prepare for the return of H1N1 virus with vaccines and other measures are essential responses to a notoriously unpredictable virus, the researchers said.