SARS

A Jupiter Scientific report on the science of SARS and how it spreads.

The Science of SARS

     The 1998 Jupiter Scientific report The Hong Kong Chicken Virus begins "Will humanity be wiped out by a deadly disease? If one day a virus emerges that can be transmitted as easily as the common cold and that has a 100% mortality rate, the answer to the above question could be yes. It is not hard to imagine an AIDS-like disease that is transmitted through the air we breathe." Was this report speaking of Severe Acute Respiratory Syndrome, otherwise know as SARS?
     The SARS outbreak is believed to have begun in November in China. SARS is a coronavirus. Coronaviruses are so named because under the microscope they have a halo or crown-like appearance. They are the source of about one-fourth of common colds and often produce mild upper-respiratory problems in humans. In animals, they are associated with gastrointestinal, respiratory, liver and neurological diseases.
     SARS is a new strand of coronavirus that was created through mutations or by combining the genetic material from two previously existing viruses. It is more likely that the latter has occurred; indeed, some researchers claim that SARS is a cross between a mouse coronavirus and an avian coronavirus. Viruses generated through genetic combination are more dangerous because the human body usually has not encountered anything like them before. As a consequence, the immune system is not as well positioned to defend the infectious attack thereby rendering such diseases more severe. SARS is the first coronavirus that is lethal.
     Will SARS become a deadly pandemic? It spreads principally through droplet transmission. When an infected person coughs or sneezes, virally contaminated material passes into the air and onto objects. If airborne droplets enter a healthy person's nose, mouth or eyes, then that person may develop SARS. Alternatively, the healthy person may touch viral material on a surface and then touch his or her nose or eyes. The best protection is to wear masks and to wash frequently one's hands avoiding contact with the face. The latter is good general advice for minimizing the chances of contracting colds and other communicable infections.

The real danger is in China and Hong Kong
where currently the number
of SARS patients is in the thousands

     Typically, there is a period T (the transmittable interval) of a day to several days during which an infected person can give a virus to another person. The rapidness by which a disease spreads is determined by a number of factors:
          (1) the number of people N₀ who are initially infected and are in the transmittable interval,
          (2) the average number of healthy people n_h that come in contact with infected people during the transmittable interval, and
          (3) the probability p_t that a healthy person contracts the disease when in the vicinity of an infected person.
If medical methods and modern science do not intervene then the number of people N(t) infected at time t grows exponentially:

N(t) ~ (TN₀/g_r ) exp( g_rt ) ,

g_r = (p_tn_h - 1)/ T

In other words, transmissible diseases with g_r > 0 should spread significantly. The growth constant g_r determines the rate of new cases.
     Because of the increase in world population, people live more closely together and n_h is greater today than anytime in the past. Two things that affect the transmission probability p_t are (a) the nature of the virus itself (some viruses such as influenza are more easily passed from one person to another) and (b) the natural immunity of some people to a virus because of genetic factors or of previous exposure to similar viruses.
     If p_tn_h can be made less than one, then the virus will not spread. Vaccines slow the growth of a disease by reducing p_t: vaccinated people are immune even if they encounter an infected person. It might not be possible to find a vaccine for SARS because coronaviruses mutate quickly. Even if it is possible, it is unlikely that it will be available this year since it takes considerable time to develop vaccines. The approach of health officials has therefore been to reduce n_h: Patients are quarantined so that they do not come in contact with others. This is manageable only if the number of people infected is relatively small. This is the situation in Australia, Brazil, France, Germany, the Philippines, Thailand, the United Kingdom, Vietnam, and dozens of other countries where only a handful of cases have arisen. In such places, the spread of SARS seems to have been thwarted. In Canada, Taiwan, Singapore and the United States, the current (May 2003) number of people sick with SARS is the order of 100 but the good news is that the number of new cases being reported is steadily going down.
     The real danger is in China and Hong Kong where currently the number of SARS patients is in the thousands and where one might not be able to quarantine everyone who is infected. A few carriers of SARS can create new localized populations of infected people, much in the way that wind-blown ashes from a forest fire can generate new brush fires. Medical authorities would then have to chase after the disease quarantining additional communities. The battle against SARS could easily get out of hand in China or Hong Kong. The World Health Organization, the Center for Disease Control, and other health organizations are like fire departments fighting a raging conflagration that threatens to spread uncontrollably to other areas.
     In remarkably rapid fashion, scientists have determined the genetic sequence of SARS. Although this will not immediately lead to a cure for the disease, it will assist in developing better diagnostic tests. This knowledge also leads to a better understanding of the disease and benefits future SARS research.
     Without preventative measures, one could easily imagine 100 million people becoming infected with SARS yearly, leading to 6 million deaths. This is what scares doctors, scientists and the informed public so much. Fortunately, appropriate steps have been taken so that one might hope that the number of deaths will end up being less than 1,000. As of May 5, 2003, there are approximately 6,600 probable cases of SARS and 460 related deaths worldwide. Using the above equation, one can estimate that the growth rate coefficient g_r is about 2 per month, meaning that each month should bring about 7 times as many new cases as the previous month if no effort to stop the spread of the disease were in place. However, the data on SARS indicate that the efforts of health organizations have already begun to slow the exponential growth in the spread of the disease. For the most recent SARS-case figures, go to the World Health Organization's SARS page and click on the appropriate link under "Highlights".

Will humanity be wiped out
by a deadly disease?

     Typically each year, between 600 million and 1 billion people around the world catch the flu, and 20,000 people die from influenza. In 1918, the Spanish Flu killed an estimated 20 million people worldwide. In the fourteenth century, 25 million Europeans, or one-fourth of the population, died from the bubonic plague, and in India and China, the disease claimed 12 million and 15 million victims respectively. On one hand, these pandemics were more contagious than SARS and modern medical methods were not available; on the other hand, airline travel makes it is easier for SARS to spread globally. This is why the World Heath Organization has proposed restrictions on certain travel.
     Initially, the symptoms of SARS are flu-like: a fever greater than 38.0°C (100.4°F), a headache and an overall feeling of discomfort that may include muscle aches and a soar throat. Within a week, SARS patients usually develop a dry cough, diarrhea and have difficulty breathing due to pneumonia. In 10 to 20 percent of the cases, respiratory problems become so acute that patients must be put on a mechanical ventilator. Somewhat less than half of such patients die. Several laboratory tests exist to detect the SARS virus.
     While SARS is not the deadly disease leading to death on a massive scale envisioned in the Jupiter Scientific report The Hong Kong Chicken Virus, it has a few of the required features. It is very fortunate that medical organizations have mobilized in they way that they have. When the next lethal viral scare comes, we may not be so lucky. The end of humanity could be just a cough away.

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