As I wrote in April when the 2009 H1N1 virus, aka swine flu, first grabbed our attention, the hallmark of the Great Flu of 1918 was that it killed the young and healthy. Early reports from Mexico indicated a similar pattern. Then the initial Mexican data was discredited as unreliable. U.S. public health officials hastily assured us that the first U.S.-Americans to die of the new flu suffered from “underlying conditions.” Never mind that those “conditions” included pregnancy – which does increase the risk of flu complications, but most folks wouldn’t consider a illness – and extremely common afflictions such as asthma.
Now the New England Journal of Medicine is reporting more systematic data from Mexico, and it’s not at all reassuring. Here’s what Gerardo Chowell and his colleagues found:
During the study period, 87% of deaths and 71% of cases of severe pneumonia involved patients between the ages of 5 and 59 years, as compared with average rates of 17% and 32%, respectively, in that age group during the referent periods.
(The full text of the study is available free online. I love the NEJM for this: they consistently provide free full content for their most important studies.)
“Familiar” seasonal flu tends to kill the very young and the very old, and the cause of death is typically pneumonia, not the flu per se. However, a series of 18 cases reported by Rogelio Perez-Padilla et al. in the same issue found that most of their patients died from the flu itself, rather than from pneumonia.
Mortality among the patients requiring mechanical ventilation was 58%, and although four patients had nosocomial pneumonia, in most of our patients, lung damage was most likely due to the primary effect of infection with influenza virus. Possible mechanisms of damage include direct injury to the respiratory epithelium with a secondary cytokine storm. We do not currently know whether our patients, especially those who died, had viremia, as was reported in association with H5N1 infection, a very aggressive variety of influenza. Coinfection with other respiratory viruses could also explain the increased pathogenicity among our patients; however, no other common respiratory viruses were found in our patients. Only three of the patients had received influenza vaccine in fall 2009, since most patients were within the age groups for which vaccine was not recommended in Mexico. It is currently unknown whether seasonal vaccination offered any protection against S-OIV infection, however. We did not find a factor that, before the onset of illness, predicted a worse outcome or death among our patients.
(Again, the whole shebang is available free online. My emphasis.)
Also, this was another young, healthy group: “More than half of the 18 case patients were between 13 and 47 years of age, and only 8 had preexisting medical conditions.” Nonetheless: “Twelve patients required mechanical ventilation, and seven died.”
This is cause for worry. Medical historians believe that most deaths in 1918 resulted from direct injury to the lungs, and they believe that cytokine storm may have played an important role.
I don’t want to be alarmist and predict a recurrence of the Great Influenza. I just want us to be ready, especially if the virus mutates to become more virulent. To that end, I’m relieved to hear that the CDC is pursing a possible mass vaccination campaign with up to 600 million doses. I’m hoping that school-aged kids will be first in line for the shots, since they’re both a vulnerable population and very efficient vectors of infection.
Oh, and I think the swine flu may have come home to my family. My brother-in-law in California has been laid low for the past several days. It looks like he’ll be fine, but the probability is high that if he’s got any kind of flu, it’s the swinish sort. I’m hoping that he’ll recover quickly and that my sister and her kids will stay healthy.