A CDC Scientist harvests H7N9 virus that has been grown for sharing with partner laboratories for research purposes. Photo: CDC

It’s been a century since the last worldwide influenza pandemic. The Spanish Influenza of 1918 spread across the globe in the waning days of World War I, killing tens of millions of people.

The next pandemic-level strain could be brewing in China, according to health officials. It’s a bird flu strain that has demonstrated 41 percent mortality in recent epidemics. Currently in its fifth epidemic, it’s become more widespread than ever – and it’s changing, according to world health officials.

The good news is that the H7N9 virus has so far shown limited human-to-human transmission. The bad news is that this fifth epidemic of the influenza strain, though mostly transmitted by poultry, is the largest yet.

“The H7N9 virus continues to have the greatest potential to cause a pandemic of known emerging influenza A viruses, and H7N9 viruses are considered to be the influenza A virus with the greatest potential public health impact,” the CDC recently announced.

Last week the World Health Organization reported 623 infections in humans,. The total cumulative number of infections to date is 1,421 since the strain was identified in 2013.

The first four epidemics proved how virulent the virus truly is, according to international authorities. From the first four epidemics, 88 percent of patients came down with pneumonia, 68 percent were admitted to an intensive care unit – and a full 41 percent died.

The fifth epidemic also appears to have diverged into two distinct lineages, from the Pearl River Delta and the Yangtze River Delta. The strain has changed ultimately at the molecular level – and for the worse, according to the CDC.

“Samples from the fifth epidemic demonstrate that these viruses contain a four-amino acid insertion in a host protease cleavage site in the HA protein that is characteristic of highly-pathogenic avian influenza viruses,” they write.

But there are positive developments. Several candidate vaccine viruses are in development. Most importantly so far, almost all the human infections have been linked to exposure to poultry, they added.

“Although some limited human-to-human spread continues to be identified, no sustained human-to-human H7N9 transmission has been observed,” they write.

CDC and the World Health Organization are not warning travelers yet. But they said that monitoring continues, and that they will issue safeguards and cautionary recommendations when appropriate.

“CDC, China and global health partners will continue to closely monitor the H7N9 virus situation in China and will continue to conduct risk assessments as the situation evolves,” they concluded. “CDC does not have any new or special recommendations for the U.S. public at this time regarding H7N9. CDC will keep you updated. Stay informed.”

This past flu season, tens of millions of birds were culled in Asia, but that was mostly due to the H5N6 strain, which was more prevalent last year than the H7N9.