Replies

He said he was surprised to find the virus in about 20 per cent of an additional 250 people who were not suspected of having SARS but who were tested because they had come to Canada from affected areas in Asia or who had mild symptoms not thought to be SARS.

So let's do the math here.... 50 people not suspected of carrying SARS are in the meantime contacting how many people over a weeks' time? Say, 150 per person? 7500 people they may have come into contact with - if 20% of these are infected, that's .... 1500 people?

Is my math right here? Do I have SARS and not know it?

If I may expand upon the math a bit

N(t) ~ (TN₀/g_r ) exp( g_rt ) ,

g_r = (p_tn_h - 1)/ T

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:

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.

Thank you.

So.... what's your gut feel? Math aside, is it time to closely examine our vulnerability, daily?

What is known about the genetic profiles of those more likely to be infected vs. those least likely? (if that is known) SARS seems to me to be a disease that somehow works via the immune system. I keep running across this CCR5-D32 thing and immune system related diseases. SARS may cue on CCR5 cytokines for cell surface binding

The equation does not take into account the quantitiy of virus parts (cells) needed to infect someone. For instance, norovirus requires as little as 10 particles to cause an infection. SARS on the other hand needs a significant number of virus parts to cause the disease. Indeed there is evidence that those that are "superinfected" go on to be superinfecters (as much as 100 people, say). A normal case might only be responsible for infecting 2 to 5 people and have a p (sub t) of 1/250. (Based on multiple cases of SARS on airlines and in schools and not transmitting)

"T" will be difficult to pinpoint since the contagious period probably begins at onset of symptoms but continues up to 10 days following recovery.

Most cases (in adults) "T" is approximately 1 month. "T" in children is as little as 12-14 days, because children seem to stop shedding as there symptoms improve.