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Medicine for Policymakers

Is H5N1 Really Highly Lethal? PDF

Eric Toner and Amesh Adalja

Biosecurity and Bioterrorism, Volume 10, Number 2, 2012. © Mary Ann Liebert, Inc. doi:10.1089/bsp.2012.0224.



How lethal are H5N1 influenza viruses to humans? The answer to this question is central to the current debate over research on genetically modified H5N1 viruses. In an effort to determine if highly pathogenic avian influenza H5N1 was capable of becoming easily transmissible between mammals, researchers in the Netherlands modified H5N1 viruses to make them transmissible between ferrets. The research has not yet been published, but reportedly the viruses were highly lethal to the ferrets when directly instilled into the ferrets’ trachea. Since ferrets are commonly used as the best animal model for simulating human influenza infection, concern has been raised that the newlymodified virus may be highly lethal and highly transmissible among people as well. Some proponents of the research argue that the H5N1 viruses are not as dangerous as they seem and that therefore concerns about an accidental release of the mutated virus or its intentional use as a weapon are overblown.1 Critics of the research argue that the currently circulating strains of H5N1 are extraordinarily lethal and that research with highly lethal viruses modified to be highly transmissible poses an unacceptable risk to society.2 In an attempt to shed light on this question, we review the available evidence.

Virus Diversity

Not all H5N1 viruses are the same. There are H5N1 viruses that are not highly lethal (low pathogenic H5N1) and that are not closely related to the viruses of concern. It is the lineage of highly pathogenic avian influenza (HPAI) H5N1 viruses that first arose in southern China in the late 1990s that is at the center of this debate. This lineage has evolved over the past 15 years into several descendant strains (clades and subclades). Clade 0 caused the original outbreak of 18 human cases (6 deaths) in Hong Kong in 1997. Clade 0 was quickly eradicated in Hong Kong and disappeared. Clades 1 and 2 are the strains that rapidly spread throughout southeast Asia in 2004. Clade 1 became endemic in Vietnam, Cambodia, and Thailand and subsequently was largely replaced by clade 2.3. Clade 2.1 has become endemic in Indonesia. In 2006, clade 2.2 spread rapidly from the Qinghai Lake region in China to much of central Asia, Europe, and Africa. There are differences in the death rates associated with these different clades. The extent to which these differences are due to characteristics of the viruses or differences in the quality and availability of health care is not known.3

Clinical Cases

As of January 24, 2012, the World Health Organization reports that 583 people have met their criteria for H5N1 infection, and of these 344 have died.4 Thus, the case fatality ratio (the number of deaths from the disease divided by the number of cases of the disease) for WHO-confirmed cases is 59%.

No one contests these numbers, but there are caveats. The WHO numbers almost certainly do not account for all H5N1 human cases and therefore might be biased, as only people who have had laboratory confirmation of H5N1 infection using WHO-approved testing are included. These case numbers reflect almost entirely people who were hospitalized. Undoubtedly there have been cases of H5N1 infection in people that did not meet WHO standards for confirmation. Also, it is likely that there were patients hospitalized with H5N1 infection without diagnosis who were never reported. Finally, there may be people infected with H5N1 infection who were never hospitalized, either because they had a mild (or asymptomatic) illness or because they died at home. It is unknown whether these uncounted people were more or less seriously ill than those who were confirmed by WHO standards and what their impact would be on case fatality statistics.

Seroepidemiologic Studies

To look for evidence of unrecognized or mild cases, a number of retrospective epidemiologic studies have been conducted. In these investigations, epidemiologists and health workers went to the families, villages, and farms where it was thought that confirmed cases had become infected. Some studies also incorporated data from healthcare workers who were exposed to case patients. They inquired about illness in the recent past and took blood specimens, looking for the presence of prior immunologic exposure to H5N1. We can find 22 such seroepidemiolgic studies of H5N1.5-26 A study of 605 non-poultry-exposed residents of Beijing in which one subject (0.17%) was found to be positive for H5 antibodies was excluded because it targeted the general population rather than those at high risk.27 The majority of these studies found little or no evidence of previously unrecognized H5N1 infections.

Four studies that did find significant numbers of people with evidence of prior H5N1 exposure were conducted in Hong Kong following the 1997 outbreak with H5N1 clade 0. Taking these 4 studies together, H5 antibodies were found in 170 of 2,368 people tested (7%).5-8 However, this clade 0 virus quickly disappeared and was substantially different from the subsequent clade 1 and 2 viruses that have caused the panzootic.3 Given that the Clade 0 virus has disappeared and was substantially different from the now-circulating Clades 1 and 2, these 4 Clade 0 studies should be excluded from the current assessment of H5N1 seroprevalence.

Excluding these 4 clade 0 studies, 11,477 people were tested in the other 18 studies, and 144 (1%) were found to be seropositive. These 18 studies used serology techniques that have limitations. Some tests are plagued by false positives, others by false negatives; some may cross-react with other influenza viruses, and there may be limited correlation among the different tests.10 In some studies, there seemed to be an association between seropositivity and exposure to untreated water, which may reflect the fact that H5N1 has been found in bodies of water contaminated by infected ducks.9,16

Two recent studies among the 18 non–clade 0 studies reported higher rates of asymptomatic H5N1 exposure than others: one among inhabitants of rural Thai villages where H5N1 was endemic, where 5.6% were found to be seropositive; and another in Vietnam that reported 5% of a high-risk cohort were seropositive.10,16 In the Thai study, seropositivity was associated with lack of indoor water— but not poultry contact—suggesting the possibility of a water-related route of exposure. The Vietnam study, in addition to using traditional hemagglutination inhibition (HAI) serology test assays, employed an ELISPOT assay to evaluate T-cell responses to H5 antigens. The HAI testing showed 5% had antibodies to H5, and the T-cell assay revealed that 3.2% reacted strongly to H5, but there was little correlation between these 2 tests—only 0.5% were positive by both tests. In addition, another 15% reacted relatively weakly to H5 by ELISPOT.10,29 The significance of these ELISPOT results is unclear. They may indicate that the traditional serologic assays are missing many people with prior H5N1 exposure, or the ELISPOT test may be picking up cross-reactions to other influenza viruses. Further study is needed.

Two researchers have recently published meta-analyses of these prior serological studies.30,31 Both concluded that, if the 4 Hong Kong Clade 0 studies were excluded and if the 13 studies that conform to official WHO criteria are analyzed, the seropositivity rate is approximately 0.5%. A total of 26 out of 5,487 people were seropositive for H5N1 in those studies.

It is important to note that these were not populationbased studies. Locations and subjects were specifically chosen because they were thought to be at high risk. Therefore, the cumulative 0.5% seropositivity rate found in these studies cannot be applied to the general population of the affected regions to infer an overall infection rate. It would be wrong to say that 0.5% of the population of these countries is infected. It is more accurate to say that in 13 studies involving more than 5,487 high-risk individuals, there is evidence of prior unrecognized infection in 27 individuals.

It is also important not to compare these results with seroprevalence studies of human influenza epidemics. Studies of these epidemics have found that mild or asymptomatic infection with human influenza is common. In human epidemics, person-to-person transmission through the respiratory tract is presumed. However, it is not at all clear how the individuals with asymptomatic seroconversion to H5 are being exposed. If, for example, some people are being exposed to contaminated water and seroconvert without symptoms, it does not neccessarily follow that people exposed to aerosolized virus would be asymptomatic at the same rate. Therefore, it is impossible to know at this time what the prevalence of asymptomatic seroconverters means in terms of case fatality rates for H5N1 viruses.

There undoubtedly are incidences of low-level environmental exposure in areas where the viruses are endemic that result in asymptomatic seroconversions, and these exposures might explain some of the findings of the recent seroepidemiologic studies. Those asymptomatic seroconversions do not necessarily predict that people exposed via the respiratory route will develop mild or no illness. It is also important to recognize that the H5N1 viruses that are currently circulating in nature are not adapted to humans. It is unknown whether a virus that did adapt to a human host and was capable of efficient and sustained person-to-person transmission, whether this occured naturally or in a laboratory, would be more or less lethal to humans than the current H5N1 viruses.

Laboratory Studies

A final way to determine the lethality of the virus is through laboratory studies. In both animal models and human cell cultures, H5N1 viruses have been demonstrated to be significantly more pathogenic than other strains of influenza. There are several factors associated with the virus and the host response that account for its pathogenicity.

Binding Properties

The hemagglutinin molecule on the surface of influenza viruses binds to sialic acid receptors in the respiratory tract of the host. Ordinary human influenza viruses bind to the alpha 2,6 form of the sialic acid receptor found in the upper respiratory tract, while avian viruses, including H5N1, preferentially bind to the alpha 2,3 form of the sialic receptor— found on the type II pneumocytes in the lower respiratory tract.32 It is thought that the ability of the H5N1 virus to bind to receptors in the lower respiratory tract is responsible for the high proportion of H5N1 patients who develop pneumonia rather than just the upper respiratory tract infection that is typically seen with seasonal influenza.

Polybasic Amino Acid Motif

For an influenza virus to infect a cell, its hemagglutinin molecule must be cleaved by a host protease. For normal human influenza viruses, this happens only in the respiratory tract. However, HPAI H5N1 contains a polybasic amino acid sequence in the hemagglutinin molecule that is not found in normal human influenza and that allows the molecule to be cleaved by a wider variety of proteases throughout the body. This feature is considered to be one reason why H5N1 is able to infect organs other than just the respiratory tract, unlike other influenza viruses.32

Viral Replication

The viral load of H5N1 has also been correlated with clinical severity. Studies have shown that the virus can replicate to a high titer in the respiratory tract, and the virus has also been found in locations outside the respiratory tract—a finding not common with other influenza strains.33 This replication reflects lack of immunity and/or immune evasion.

Cytokine Storm

In response to infection with H5N1, host cells are known to secrete higher amounts of inflammation-influencing cytokines than with other strains of influenza. It has been shown that the H5N1 polymerase genes (PB-1, PB-2, PA) and the NS gene segment induce this hypersecretion. Specific cytokines that increase in response to H5N1 include IL-10, IL-6, and IFN-γ.32 These cytokines cause many of the clinical manifestations of the disease, and it is suggested that excess secretion of cytokines—a cytokine storm—is one cause of the high fatality rate.


While there is uncertainty and many unanswered questions, in our judgment most of the concrete evidence to date does not support the notion that there is a broad spectrum of clinical severity associated with H5N1 infections in humans. It appears that most people who become ill from these viruses are very sick indeed and that an unusually large proportion of these people die. Despite concerted effort to find mild cases, very few have been documented. There undoubtedly are incidences of low-level environmental exposure in areas where the viruses are endemic that result in asymptomatic seroconversions, and these exposures might explain some of the findings of the recent seroepidemiologic studies. That does not mean, however, that the severity of illness would be less in people who receive a dose of the virus sufficient to cause illness via the respiratory route.

Overall, the preponderance of the evidence to date from clinical, seroepidemiologic, and laboratory studies supports the notion that HPAI H5N1 viruses now circulating are extraordinarily lethal to humans compared to other influenza viruses.


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