The influenza pandemic of 1918-1919, often called Spanish flu, is still a topic of medical research because of concerns that a similar epidemic could occur again. Dr. Jeffery K. Taubenberger and Dr. David M. Morens of the National Institutes of Health provided a review. These authors called the Spanish flu pandemic “the single most fatal event in human history.” The estimated deaths were at least 50 million worldwide, and many of the victims were previously healthy young adults.
Medical scientists of 1918 were not able to isolate the influenza virus, but they preserved autopsy specimens. In 1995, researchers began the process of reconstructing the Spanish flu virus. Over the next decade, they obtained a complete genome sequence. The 1918 virus was type H1N1, based on the variety of hemagglutinin (H) and neuroaminidase (N) proteins on the surface. H1N1 viruses have continued to circulate in the human population, but they change every year.
Taubenberger and Morens reported that the source of Spanish flu was probably avian, but no one knows how or where the virus first infected humans. Thus the name “Spanish” is arbitrary. “It is now believed that the pandemic virus appearing in 1918 was transmitted from humans to pigs early on,” they added. In 2009 descendants of the 1918 virus caused a milder pandemic sometimes called “swine flu”. This H1N1 virus affected both humans and swine, but it is not clear, Taubenberger and Morens noted, which species developed it first.
Secondary bacterial pneumonia is recognized as the cause of most deaths in previous pandemics. “Circulation of clinically aggressive community-acquired methicillin-resistant Staphylococcus aureus is an additional factor to be considered in planning for pandemic response,” Taubenberger and Morens warned.
Dr. Tadanobu Takahashi and colleagues in Japan and Wisconsin studied molecular characteristics of the 1918 Spanish flu virus. They found the virus is stable in acidic conditions of low pH, and suggested this is one of the reasons for its exceptional ability to cause human disease.