H7N9 Situation and CDC Response Update

May 10, 2013 – Sporadic human cases of infection with avian influenza A (H7N9) continue to be reported in China, but the increase in case counts has fallen off from the month of April, when multiple cases were being reported each day. As of the latest report on May 8, there were 131 confirmed cases of H7N9 with 32 deaths. Between April 1 and April 29, 126 cases had been reported, including 24 deaths. China and WHO have switched to weekly reporting of H7N9 cases. CDC continues to take proactive preparedness measures in response to the public health threat posed by the emergence of this new virus.

The drop-off in newly reported H7N9 cases in China may be the result of containment measures reportedly taken by Chinese authorities, including closing live bird markets, a venue where the risk of exposure to bird flu viruses can be high. However it may also be a result of changing seasons, or a combination of both. Studies indicate that avian influenza viruses have a seasonal pattern to them, much like human seasonal influenza viruses. After watching H5N1 for more than a decade, it’s clear that activity in poultry – and resulting human infections – increase in the winter and decline in the summer. It’s possible that the same will be true with H7N9 and that as H7N9 activity in poultry or other birds declines, so will the chances for human exposures and resulting human infections. If this is the case, H7N9 infections – in birds and people – may pick up again when the weather turns cooler.

While the number of human infections being reported has dropped, the epidemiology of the current outbreak has not changed significantly. Evidence continues to point to infected poultry or contaminated environments as the source of most infections. There is still no evidence of sustained (ongoing) human-to-human transmission with this virus and no cases have been reported outside of China.

CDC does not have any special recommendations for the U.S. public associated with H7N9. In addition, CDC does not recommend restricting travel to China at this time. However, travelers to China should practice hand hygiene, follow food safety practices (including eating only meats and poultry products, including eggs, that have been cooked thoroughly), and avoid contact with animals. Visit www.cdc.gov/travel for more information.

While the risk to people in the United States from H7N9 continues to be low, because of the pandemic potential posed by this virus, CDC is following this situation closely and coordinating with domestic and international partners, including the Chinese Center for Disease Control (China CDC) and the World Health Organization (WHO). CDC also is taking proactive steps to be ready for the possibility that this virus may become fully transmissible between people. 

Shipping of the CDC-developed H7N9 diagnostic test kits began on April 25, 2013. By May 10, 2013 122 test kits had been shipped; 95 of these domestically and 27 of them internationally. At this time, test kits have been shipped to 47 U.S. states and the District of Columbia. Each kit is able to test about 1,000 specimens for H7N9. Public health laboratories are being asked to rule out seasonal influenza before they use the H7N9 test kits. CDC’s seasonal flu diagnostic tests will produce an influenza A “unsubtypable” result on an H7N9 sample. The new kits will allow H7N9 to be detected much more rapidly than if all “unsubtypable” samples had to be sent to CDC for testing, thus enhancing the country’s ability to quickly detect this virus in the U.S.

While no cases of H7N9 have been detected at this time in the U.S., 54 people with flu-like symptoms after travel to China have been tested. All have 54 tested negative for H7N9; while 6 tested positive for seasonal influenza A, and 3 tested positive for seasonal influenza B.

Should a case be detected in the United States, this would not signal an increase in the potential risk to the public’s health unless the transmission pattern of the virus was to change. If a person in the United States has H7N9 flu, they will be isolated (separated from other people who are well) and cared for. In addition, a contact investigation will be done with people who may have been exposed to the sick person.

Contact investigations are one of the ways CDC works with partners in the United States and other countries to protect the health of people exposed to an illness. This process involves finding, interviewing, and in some cases, testing or treating the people who came into contact with the sick person. All of this can help confirm that the virus is not spreading further in the United States.

At this time, a decision to produce H7N9 vaccine for a U.S. national vaccination response has not been made. However, given the severity of human illnesses from H7N9 in China, HHS and its partners are taking preparedness steps to develop H7N9 candidate vaccine viruses and are planning for H7N9 vaccine clinical trials.

CDC is working on developing candidate vaccine viruses (CVVs) from two different H7N9 isolates (Shanghai/2 and Anhui/1). A CVV is a flu virus that CDC (or one of the other WHO Collaborating Centers) selects and prepares for use by vaccine manufacturers to make a flu vaccine. These CVVs could be used to manufacture vaccine if one is needed. Candidate vaccine viruses are typically chosen based on their similarity to flu viruses spreading and causing illness in people as well as their ability to grow easily in chicken eggs, which is the primary method of manufacturing influenza vaccine. Without a high-yield candidate vaccine virus, it can be very difficult to manufacture vaccine to protect against a new influenza virus.

Once a vaccine manufacturer receives a candidate vaccine virus, manufacturers then create what is known as a "seed strain." The seed strain is adapted by each manufacturer to make the virus grow better using their technology and production systems. Once the seed strain is prepared, the vaccine manufacturer uses it to grow large quantities of virus for producing flu vaccine.

On May 1, 2013, CDC offered to begin shipping potential candidate vaccine viruses to qualified laboratories with biosafety-level 3 facilities that wanted to begin working on creating their own seed virus early. These are candidate vaccine viruses that still require some in vitro and in vivo studies to be completed to meet full regulatory and biosafety requirements. Other WHO partners have potential candidate vaccine viruses as well. Neither the World Health Organization or the U.S. Food and Drug Administration have made any recommendation about which H7N9 potential candidate vaccine virus is recommended for use in the manufacture of H7N9 vaccine.

It’s important to note that influenza vaccine production is complex and can be unpredictable and has many critical and time-sensitive steps; delay at any point during these steps can result in delays in the availability of influenza vaccine. However, it usually takes about six months to produce large quantities of influenza vaccine.

CDC will provide updates as more information becomes available. More information about H7N9, including a Frequently Asked Questions document, is available at Avian Influenza A (H7N9) Virus.