Will vaccine protect HCWs before a pandemic?
Researchers seek partial immunity from vaccine
Could health care workers receive some vaccine protection from avian influenza even before a global pandemic occurs?
Researchers are investigating whether an H5 vaccine could provide “priming,” or partial immunity, that would strengthen a response to future, more specific vaccines. If so, world health experts would consider the possibility of vaccinating health care workers or other high-priority groups prior to a pandemic, but the safety of the vaccine would have to be clearly established, said Margaret Chan, MD, the World Health Organization’s assistant director-general for communicable diseases.
“We have to be absolutely clear that in the absence of an H5N1 outbreak affecting humans in big numbers we need to look at the trade-offs. If we are not going to do good, we would not wish to do harm,” she said at the International Conference on Emerging Infectious Diseases held in Atlanta in late March.
Chan warned that the potential for H5N1 pandemic influenza is greater than ever, within its rapid global spread among wild birds and poultry and genetic evolution of the virus. It took two years for H5N1 avian influenza to spread to 15 Asian countries. But in the past three months, a time of intense surveillance, the virus was detected in another 20 countries in Asia, Europe, and Africa, she noted.
As of March 21, there were 184 human cases identified in eight countries, with 104 deaths. There has been no evidence of sustained, human-to-human transmission.
“The pace with which it has spread is reason for concern,” said Chan. “We have never seen this in the history of avian influenza. We are receiving reports of sporadic human infection. . . The risk of pandemic is great.”
Two distinct genetic strains of H5N1, termed “clade 1” and “clade 2,” have caused infection in humans. Animal studies have shown that the two strains are distinct enough to require a separate vaccine, said Nancy Cox, PhD, chief of the influenza branch of the Centers for Disease Control and Prevention (CDC).
“What we have found so far is that there is some cross-reactivity, but it certainly isn’t at the level we would like,” she reported.
Cox acknowledged that the virus could continue to mutate beyond the two clades. Clade 1 caused human disease in Vietnam, Thailand, and Cambodia in 2003, and clade 2 was identified in human cases in Indonesia in 2005.
“The expectation is that the immunologic priming of an individual with an H5 vaccine, whether there was a perfect match or not, would provide some measure of protection,” she said. “That assumption is not proven, but it’s based on what we’ve seen with the regular seasonal influenza viruses when they undergo antigenic drift.”
Mask recommendation could change
Meanwhile, world health experts acknowledge that recommendations about mask use could change as they learn more about the disease.
Decisions about respiratory protection hinge on the science of influenza transmission — whether it is airborne or droplet-borne, said Marty Cetron, MD, director of the CDC’s Division of Global Migration and Quarantine.
“The science around it is uncertain. It’s an area of active, ongoing debate,” he said. “The guidance is dynamic.”
Currently, the CDC recommends using surgical masks as barrier protection against influenza — whether it is seasonal or pandemic. Higher levels of protection, such as an N95 filtering facepiece respirator or powered air-purifying respirator (PAPR), would be appropriate for protection during aerosolizing procedures, the CDC says.
The WHO recommends the use of respirators such as the N95. “HCWs working with [avian influenza]-infected patients should select the highest level of respiratory protection equipment available, preferably a particulate respirator,” its guidelines state. “Surgical and procedure masks do not offer appropriate respiratory protection against small-particle aerosols (droplet nuclei) and should not be used unless particulate respirators are not available when dealing with airborne transmitted diseases.”1
1. World Health Organization. Avian influenza, including influenza A (H5N1) in humans: WHO interim infection control guideline for health care facilities. Geneva; Feb. 9, 2006. Accessed on-line at: http://www.who.int/csr/disease/avian_influenza/guidelines/infectioncontrol1/en/index.html.