James Martin Center for Nonproliferation Studies
Combating the spread of weapons of mass destruction with training & analysis

CNS Occasional Papers: #9

by Janet R. Gilsdorf, M.D.

Background

Smallpox epidemiology

The epidemiology of smallpox varies with the vaccine status of the population [1]. In areas where smallpox has never occurred, the attack rate is high among all ages and the death rate is highest among older children and young adults, as is the case with the introduction of other viral infections such as varicella and measles into naïve populations.

In endemic areas, children experience higher attack rates, because adults are immune by virtue of past infection.

In populations with active programs of childhood vaccination, young children have low attack rates, because most have been recently vaccinated, while adults have higher attack rates, reflecting their waning immunity.

In populations with recent lapses in childhood vaccination programs, the attack rate in children is relatively high because many are not vaccinated, and relatively low among adults, reflecting immunity from past immunization.

In the Aralsk outbreak, the population was not well vaccinated, and the vaccine status appears to be worse among younger children than among older children (see Table 1).

These data suggest that immunity of the Aralsk population was lowest in younger children, probably reflecting a fall-off in vaccination rates among infants. They also suggest that previous vaccination had not been highly effective. Furthermore, the Aralsk report cites 10 students in patient #2's classroom who had vaccine scars and yet developed a primary take on revaccination; the number of children examined in this classroom was not given, so the rate cannot be calculated. More information on the efficacy of the vaccines and vaccine strategies used prior to 1971 in Kazakhstan would be critical to understanding the meaning of the Aralsk outbreak.

Thus, in 1971 in Aralsk, the vaccine coverage of the population was sub-optimal (certainly low enough to sustain an outbreak until it was by halted widespread vaccination and quarantine) and the efficacy of the past vaccine must be questioned. This concern is further highlighted by patient #4 who developed classic smallpox in spite of having received a vaccine 2½ years earlier; either the vaccine was of low potency or was different enough antigenically from the infecting strain to render only partial immunity. The other information on poor immunity after prior vaccine supports low potency of the vaccine.

Smallpox virulence

The virulence of variola is dependent on both host and viral factors, which act in concert to determine the outcome of the host-virus interaction. Host factors include:

1. Acquired immunity, resulting from
   1. Transplacental antibody, which appears to be short-lived, providing strong immunity for the first month after birth and partial immunity for the first four to six months after birth [2].
   2. Passive immunity, from infusions of hyperimmune serum from recipients of smallpox vaccine and was not an issue in this outbreak as it was only used to treat [unsuccessfully] infected individuals with serious disease.
   3. Vaccine-induced immunity, which varies on the ability of the host to mount a robust and specific immune response to the vaccine, on the potency of the vaccine (as a live virus vaccine, it requires fairly rigorous storage to preserve potency), on the time since vaccination or re-vaccination, and on the antigenicity spectrum of the vaccine as compared to an outbreak strain.
   4. Past infection, which may not have been an issue in this outbreak since endemic smallpox may not have occurred in this region for some time.
2. Innate immunity, which dictates a host's ability to mount an effective response to the vaccine and to mount a protective response to contain variola infection.
3. Unknown host factors that alter the host's response to vaccine or infection, reflecting, for example, polymorphisms in T-cell receptors or genetic variations in immune signaling.

Viral factors include:

1. Exposure load
   Household contacts in close proximity to infected individuals are exposed to a higher concentration of virus, thus increasing their likelihood to become infected. Similarly, patients are most contagious at the time they are excreting the highest levels of virus. One aspect not addressed in the report is the other crew members of the research ship. Apparently, none of them developed smallpox. Why? It may be as simple (or complicated) as the two women in the U.S. anthrax outbreak for whom no exposure could be identified. The best way to explain those situations is that the level of viral exposure necessary to infect a host is not finite, but varies to include a relatively low level in a very few individuals.
2. Strain to strain differences in pathogenicity
   Possible variations in pathogenicity of variola strains must be considered in the context of outbreaks rather than in the context of individual patients, because within any outbreak, variation in the severity of the disease is striking and dependent on many factors. The most compelling evidence for strain to strain differences in variola pathogenicity is the existence of geographically defined outbreaks caused by variola major strains (with mortality rates of 5-40%) and variola minor strains (with mortality rates of 0.1 - 2 %) [3]. Biological differences between strains associated with variola major or variola minor have been described, such as differences of haemabsorbtion by infected human cells [3] suggesting these viruses induce different red cell receptors in cells they have infected, pathogenicity for chick embryos, and temperature sensitivity. The relevance of the animal tissue toxicity tests by Sarkar and Mitra [4] remains unknown, but the concordance of toxicity with severity of human disease among the strains tested is intriguing. Rough genetic analyses of viral strains have identified truncated peptides and strain to strain differences in restriction patterns [5], but these studies have not identified viral factors clearly associated with more severe disease. More sophisticated genetic studies weren't possible when epidemic smallpox was still circulating and, since studies to explore viral genetic traits associated with increased virulence must be done in the context of careful epidemiological studies stratifying patients by clinical outcome, viral virulence factors aren't likely to be defined anytime soon.

QUESTION 1: Is Dr. Zelicoff's analysis correct that the source of the Aralsk smallpox outbreak was a field test of smallpox on Vozrozhdeniye Island?

The timing of the initial case and the secondary cases are consistent with this hypothesis. The only uncertainty is the possibility that Patient 1 was indeed exposed to smallpox from another source during the boat trip around the Aral Sea. Since the former (and present) governments of the region have many incentives NOT to report smallpox outbreaks (and did not report this one), relying on the official reports to discount another type of exposure may be misleading.

QUESTION 2: Do the data presented in the Soviet report indicate that the causative virus strain was weaponized to be especially virulent and/or vaccine resistant?

Virulence

Increased virulence is a multifactorial characteristic that may result from increased transmission or increased pathogenicity of the virus. Increased transmission may involve two different scenarios:

1. Expanded distribution of the virus after an event of mass exposure, such as a terrorist attack or an accidental exposure from a common source.

   Dr. Zelicoff presents compelling data to support the hypothesis that the strain initiating this outbreak was able to infect individuals farther from the source (15 kilometers) than previously thought possible.

2. Increased transmission from person to person.

   This outbreak does not present data addressing this possibility, as the contacts are poorly described in terms of vaccine status or degree of contact. A virus that had been formulated to increase its survival after a point exposure may not continue to be transmitted from person to person at an increased rate.

Dr. Zelicoff's hypothesis of increased pathogenicity of the virus rests on the observation that in the Aralsk outbreak three of the ten cases (one adult and two infants, 4 and 9 months of age) -- the three unvaccinated cases -- died and had the hemorrhagic form of smallpox. He cites evidence from the studies of Rao in India that the hemorrhagic form is rare in infants, which is enigmatic considering that the death rates in patients studied by Rao are highest in young children [2]. The diagnosis of hemorrhagic smallpox may be subjective and open to question; for example, plate 2 in Dr. Rao's book [2] is labeled flat type smallpox lesions on day eight of illness and yet many of the lesions appear hemorrhagic, consistent with the late type of hemorrhagic smallpox (as opposed to the early type, which is purpuric, most likely reflecting a generalized bleeding diathesis that occurs just as the rash is emerging). Since so many questions surround the designation "hemorrhagic" smallpox, a better endpoint to consider as a measure of serious disease is death.

Although the role of host susceptibility versus viral pathogenicity in the highly fatal hemorrhagic form remains unresolved, two pieces of data support the fact the hemorrhagic disease is the result of host factors.

1. The hemorrhagic form occurs in equal proportions among patients with variola major and in patients with variola minor, suggesting that the hemorrhagic form of the disease does not correlate with strain pathogenicity as reflected by death rates associated with the variola major and variola minor strains of the virus.
2. The hemorrhagic form occurs in equal proportions among vaccinated and non-vaccinated patients. The interpretation of this observation is difficult, but it may reflect the hemorrhagic form occurring in patients with altered T-cell function who don't generate optimal immunity from vaccine and don't contain the virus well when infected with either variola major or variola minor (a so-called lacular defect).

The hemorrhagic forms of smallpox need to be better understood in light of the host innate immune system, possibly using meningococcemia as a model. The various clinical forms of meningococcal disease, ranging from fever and bacteremia with or without petechial rash through bacteremia and meningitis to overwhelming sepsis with disseminated intervascular coagulation and death, do not appear to be strain dependent.

Vaccine resistance

Microbial antigenic variation, in which a vaccine that contains antigens from one strain provides only partial immunity against other strains, is well known for viruses that recombine readily with related viruses, the most infamous being influenza virus. We know too little about variola and its interactions with other poxviruses to suggest a mechanism for antigenic shift and too little about its natural genetic variation to support significant antigenic drift. Other potential (and probably more likely) explanations for vaccine failure are poor quality of the vaccine, host inability to mount an optimal viral-specific immune response, or waning immunity with time since vaccination.

Thus, the Aralsk report includes too few patients and too little information on the vaccine status of the contacts and their degree of contact to the cases to make strong statements about transmissibility, virulence, or vaccine resistance of the virus.

QUESTION 3: Does the Soviet report have implication for international biological arms control?

While the Soviet report and Dr. Zelicoff's analysis do not prove that this outbreak represented exposure to a hypervirulent or particularly hearty virus, they do remind us that we understand very little about the activities of the former Soviet Union in developing biological weapons and would be wise to consider many possibilities.

QUESTION 4: Does the Aralsk outbreak have implications for the development of a national smallpox vaccine strategy?

No one familiar with the currently available smallpox vaccine (prepared using 1950s or earlier technology) would be satisfied with this vaccine as the sole method of preventing smallpox should outbreaks be likely. The Federal Government appears poised to devote significant resources to improve our understanding of variola and its potential pathogenic and immunogenic factors, with the goal of developing safer, yet highly effective, vaccines. In this context, information concerning variola antigenic variation, both naturally acquired and biologically engineered, is critical to developing newer vaccine components.

References

1. Banks, H.S., Smallpox, in The Common Infectious Diseases. 1945, Edward Arnold & Co.: London. p. 209-223.
2. Rao, A.R., Smallpox. 1972, Bombay: The Kothari Book Depot. 220.
3. Fenner, F., et al., Smallpox and Its Eradication. 1988, Geneva: World Health Organization. 1460.
4. Sarkar, J.K. and C. Mitra, Virulence of Variola Virus Isolated From Smallpox Cases in Varying Severity. Ind. Jour. Med. Res., 1967. 55(1): p. 13-20.
5. Dumbell, K.R., et al., A variant of variola virus, characterized by changes in polypeptide and endonuclease profiles. Epidemio Infect, 1998. 122: p. 287-290.

Return to Occasional Paper #9.