A fascinating report from CIDRAP by Robert Roos: Study suggests 1918 flu waves were caused by 'distinct' viruses. Excerpt
A new analysis of influenza outbreak patterns in several navies in 1918 suggests that the first and second waves of the pandemic were caused by "antigenically distinct" viruses, a conclusion that runs counter to some other recent studies that have tried to discern why the second wave was so much more deadly than the first.
A team of Australian and American researchers, writing in The Lancet Infectious Diseases, says its analysis suggests that those who were infected in the mild first wave, in the spring or summer of 1918, remained susceptible to infection in the deadly second wave in the fall, but were protected from death.
If this is true, "the two waves of infection were probably caused by antigenically distinct influenza viruses—not by one virus that suddenly increased in pathogenicity between the first and second waves," says the report by G. Dennis Shanks, MD, of the Australian Army Malaria Institute, and colleagues.
This conclusion contrasts with a study published last week, in which US researchers examined preserved lung specimens from 68 US soldiers who died in the pandemic between May and October of 1918. They found that earlier and later cases featured the same type of lung damage, and they concluded that the pandemic virus was circulating by May.
The new findings also contrast with those of a 2008 study that covered similar ground. In that one, US researchers examined data mainly from US Army camps and found that soldiers who were infected in the first wave were both less likely to be infected and less likely to die in the fall onslaught of flu.
Sifting naval records
Researchers have been trying for years to explain the puzzling pattern of the 1918 pandemic, in the hope of drawing lessons for pandemic preparedness today. The Lancet report notes that the 1918-19 pandemic had three waves: one in the spring and summer of 1918 that was "highly infectious but clinically mild," one in the fall that was highly infectious and caused far higher mortality, and one in the winter of 1919 that was smaller and less deadly than the fall episode.
"The three waves of infection are often assumed to share the same viral cause; however, little objective evidence supports this notion," the report says.
The researchers studied illness records for the crews of ships in the British, Australian, US, and Japanese navies in 1918, and also looked at morbidity and mortality records for some island residents. They focused on ships' crews because of their homogeneous composition, isolation at sea, and consistent record keeping.
Nearly all ships had flu outbreaks, but few had deaths, the team found. However, the ships that did have deaths tended to have many, mostly during the second wave. Citing previous work, the authors say an estimated 1.1% of US Navy personnel died during the pandemic.
One ship that exemplified the general pattern was the USS Salem, which patrolled the Caribbean, according to the report. In February 1918 the ship had a flu outbreak that affected many of the crew but caused no serious complications. Then in September and October, during the second wave, 13% of the crew got sick, but all survived.
A contrasting example was the USS Pittsburgh, which patrolled the Atlantic off South America in 1918. When the ship docked in Rio de Janeiro in early October, 663 sailors—about 80% of the crew—contracted flu, and 59 of them died. Those deaths accounted for nearly half of all flu-pneumonia deaths on US Navy cruisers that year.
"In general, naval warships that had high mortality during the second wave were not affected by the first wave because of their relative isolation (eg, in the southern hemisphere)," the report says.
US military personnel in Hawaii offered another example, the researchers found. Many of the sailors and soldiers there had a mild flu-like illness in mid-1918. Later in the year, when the second pandemic wave hit, only one US sailor in Hawaii died of flu and pneumonia. A similar pattern was seen in a US Army camp in Puerto Rico.
Click through to the CIDRAP page to read the whole article and access the Lancet abstract.
