October 22, 2018
4 min read

Human cases of H7N9 plummet in China after ‘aggressive’ vaccination of poultry

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In China, human cases of avian influenza A(H7N9) plummeted to nearly zero during the sixth epidemic of the virus following an “aggressive” campaign to vaccinate chicken and other poultry, experts said.

According to officials, there have been 1,567 laboratory-confirmed human cases and 615 deaths because of H7N9 since March 2013, when the strain was first detected in people. The fifth wave of H7N9 was the largest by far, with 766 reported cases, leading experts to worry about what the sixth wave would bring. In the past year, however, only three human cases of H7N9 were reported — a decline that coincided with China vaccinating “billions” of chicken and other poultry, CDC Principal Deputy Director Anne Schuchat, MD, noted during a keynote address at ASM Microbe in June.

“We have no sense of why the fifth [epidemic] was so high. There was just no reason that we can come up with beyond the spread in poultry,” Michael T. Osterholm, PhD, MPH, director of the Center for Infectious Disease Research and Policy at the University of Minnesota, told Infectious Disease News. “As far as the past year, clearly the Chinese began a very aggressive and comprehensive program to vaccinate poultry against H7N9. That is by far the most likely explanation for what happened, but even there we can’t be certain.”

Vaccination campaign

H7N9 viruses have not adapted to be easily transmitted by humans, and most infections reported in people have spread from poultry. But health experts worry about the ability of H7N9 viruses to cause a pandemic. Data from China show the virus kills about 40% of infected patients who are hospitalized — although the actual mortality rate is likely lower, experts have said — and a CDC tool that evaluates the risk posed by influenza A viruses has determined H7N9 as the strain likeliest to cause a pandemic.

Michael T. Osterholm

According to WHO, reports from China have shown that there have been fewer H7N9 virus detections in poultry and environmental samples during the sixth epidemic. In a recently published study in Cell Host & Microbe, researchers from the Chinese Academy of Agricultural Sciences reported that H7N9 infections in poultry markets and farms “dramatically decreased” following the introduction of the inactivated vaccine in poultry — mainly chickens — starting in September 2017. (The authors also noted that many infected poultry were slaughtered, and that evidence shows genetic variations of H7N9 subsequently appeared in ducks.)


Osterholm said the prevailing explanation for the drop in human infections is that it is tied to the vaccination campaign in poultry, although he has not seen data to confirm this.

“In that case, it surely would be a major success story,” Osterholm said.

Questions remain. Among them, Osterholm said, are whether the vaccine eradicated H7N9 in poultry or suppressed it, and if it will re-emerge genetically different.

“The verdict is still out on if this is a great public health success story,” Osterholm said. “Surely, it appears to have had an impact for 1 year. What we don’t know is what that impact will be like over time if this continues.

H7N9 samples

In 2017, the United States was forced to make new vaccines for H7N9 after evidence emerged that the virus had split into two strains, making the old vaccine stockpile useless. Trials of a new vaccine candidate began near the end of 2017.

In a more recent development, amid an escalating trade war between the two countries, The New York Times reported in August that the U.S. had not received important lab specimens of H7N9 from China in more than a year — a type of exchange that has been routine under rules established by WHO for influenza strains with pandemic potential, according to the report. However, a spokesman for HHS confirmed to Infectious Disease News that the CDC has received five of eight virus samples that were requested from China and is awaiting delivery of the remaining three virus samples.

Osterholm said if it is shown that a mass vaccination of poultry reduced H7N9 transmission in humans, it would represent “a major breakthrough in terms of controlling these unique strains of virus in poultry around the world from causing human illness, and would serve as an important public health model for what to do.” He said the best example of this type of program is vaccinating cats and dogs against rabies, a front-line intervention to prevent the disease in humans.

There have been no cases of H7N9 reported in the U.S. If an animal were to be infected with H7N9, there are no effective antivirals to treat it, Osterholm said. For humans, the primary drug of choice would be Tamiflu (oseltamivir, Genentech), if the infecting strain is sensitive to the antiviral, he said. He added that several other new antivirals being evaluated also could be possible treatment options. – by Bruce Thiel and Gerard Gallagher

Disclosure: Osterholm reports no relevant financial disclosures.