May 29, 2013

Population immunity to H7N9 very low

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The population’s immunity to influenza A(H7N9) is similar to that observed for other avian influenzas, which is generally very low, researchers have found.

As of May 24, there have been 131 cases of influenza A(H7N9) in China. There also have been 36 deaths, according to WHO.

“On the one hand, it is reassuring that we do not see any evidence of past H7N9 infections, indicating that the current outbreaks do not represent a tip-of-the-iceberg phenomenon of widespread H7N9 infection in people,” Maciej F. Boni, PhD, of the Center for Tropical Medicine, Oxford University Clinical Research Unit in Ho Chi Minh City, Vietnam, told Infectious Diseases in Children. “On the other hand, the low antibody levels we measured indicate that there is likely to be very little immune protection against this virus.”

Maciej F. Boni, PhD 

Maciej F. Boni

Boni and colleagues evaluated 1,723 serum samples collected prospectively from southern Vietnam. They tested the sera for antibodies to five different avian influenza antigens. One of the antigens was A/chicken/Netherlands/1/2003 (H7N7), whose HA1 protein is similar to that of the earliest H7N9 strain sequenced in the current outbreak. They also screened for antibodies to one H9 antigen and three H5 antigens, and antibodies to 11 human influenza viruses.

Among the avian influenza viruses, they found that antibodies for the H7 antigen were at higher titers than those for H5, but titers for H9 were still higher. Antibody titers for human influenza viruses were the highest. The antibody titers increased with age and with increasing antibody titers to human influenza viruses.

“It is now essential that follow-up studies on patients with H7N9 include serological investigations to determine what antibody levels are indicative of clinical protection and what antibody levels are indicative of past infection,” Boni said. “These data will give us a better tool to gauge the size of a second H7N9 outbreak should this virus re-emerge in the future.”

For more information:

Maciej F. Boni, PhD, can be reached at Oxford University Clinical Research Unit, Hospital for Tropical Diseases; 764 Vo Van Kiet St., District 5, Ho Chi Minh City, Vietnam; email:

Disclosure: Boni reports no relevant financial disclosures.