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Thursday, June 18, 2009

Swine Flue - Keep it weak

Medical Officers don't stand a chance. (New Scientist, 20Jan, 2012 p28) If they stop a pandemic dead in its tracks, they will always be accused of overreacting.  If one gets away from them they will be accused of too little too late.  It is very likely that the very act of quarantining flue victims and their contacts is what avoided an H5N1 disaster.  A flue strain is deadly when it attacks the body so quickly that the body doesn't have time to create the appropriate antibody, a process that takes a week or so.  If you quarantine a victim with a deadly flue strain and his contacts as soon as he shows symptoms, he can't pass his variety of flue on to others.  In the mean time, people with more benign mutations of the same H5N1 virus that doesn't create early harsh symptoms and that  the body can handle are infecting others.  Any strain you contract of the same virus gives you immunity to all strains.  The next time the same flue comes around, from the viruses  'point of view', there are far less people for it to infect.  With an effectively reduced population, there is an added selective pressure for flue varieties that are more benign and can infect other victims before symptoms show in the infector.  This is likely the explanation why deadly diseases become less deadly the more time they are in contact with their human population.  We have to stop accusing the authorities of over reacting and continue to quarantine flue carriers and their contacts as soon as they are detected.  The more of us there are and the more crowded together we are, the more important this becomes.

It is reasonable to believe that the actions being taken around the world to limit the spread of swine flue is keeping it from becoming lethal.

When the flue virus enters a new human host, it takes over the genetic replication mechanism of some of the host cells and uses them to produce more virus particles. In the mean time, the body begins to ramp up its immune system to produce antibodies to kill the virus. The virus has to spread to a new host before the host produces enough antibodies to kill it or before the host dies. Typically antibody production takes about a week. The virus is therefore more successful (spreads to more hosts) if it spreads easily and if the host remains infectious and in contact with oters for as long as possible. To remain infectious, the host has to remain alive. Not only do viruses die in a dead host but a dead host is not moving around infecting new hosts.

Once a new virus has mutated to solve the problem of transfer from human to human, it begins to spread. The virus is not consciously motivated, of course and some of the viruses will mutate into a deadlier or less deadly form completely at random. Its chance of mutating depends on how many virus particles there are. If only a few humans are infected, there is less of a chance that a virus will mutate than if there are huge numbers of carriers. This is the first way in which limiting the spread of the virus mitigates against it mutating to a more lethal form. By limiting the number of humans infected, you limit the number of viruses available for mutation and hence you reduce the chance of a lethal form popping up.

However, given time, some viruses will mutate into a deadly form. Lets say for the sake of argument the frequency is one in a billion billion billion viruses. When such a virus infects a person, the symptoms will be severe and the person is likely to die. Here is the second way that slowing down the spread of the virus keeps it weak. If every time someone has visible symptoms, he is isolated, he will get better or die without his variety of the virus spreading to a lot of other hosts. Isolate his contacts and you further limit the chance that a lethal variety will spread. By killing its host the virus "burns itself out". In fact, the only virus that can spread freely is one that is mild enough not to force the host to stay quarantined. The ultimate, genetically successful virus would be completely asymptomatic.

So varieties of the virus which manifest themselves with mild or even no symptoms will stay out there spreading through the population. Here is the next reason why slowing the spread of the virus reduces its lethality. The longer you keep nasty varieties from spreading, the more time there is for the mild varieties and even asymptomatic varieties to spread. Remember that these mild varieties are also imbuing their hosts with immunity to this whole cohort of viruses (in the recent case, the H5N1 of 2009). Here there are two effects. First, a host that has been infected by a mild variety won't later contract a deadly variety and secondly, the more people that are immune, the greater the herd immunity and the slower the virus will spread when it comes through the second time.

In a thin host population, any virus which becomes deadly will burn itself out. By killing it's host, it kills itself. If it is killing the host before it can infect at least one and a bit new hosts, then it will disappear and only more benign forms of the virus will carry on - namely those which allow the host to live long enough to infect more people. In our modern world, we are particularly vulnerable. We are very crowded which gives many opportunities for the virus to spread to new hosts and methods of transport give many opportunities for the establishment of new foci in other geographical locations. Remember, that if the person can be kept isolated for a week or so, they will no longer be infectious. Then another factor comes into play.

Here and there, though, a virus will arise by mutation that is deadly. It will kill its host quickly. If we take no precautions to slow its spread, in a crowded population, a huge number of people can become infected before the first person dies. If, however, we take extraordinary precautions when someone dies from the virus, regardless of what we think of their vulnerability, we will tend to wipe out the deadly form. Extraordinary measures, of course, consist of trying to contact all the contacts of the victim and getting them to isolate themselves for a week. Of course with modern medicine; with life support systems and antibiotics to protect against secondary infections, mortality for the isolated people is much less likely than it once was.

And how about immunization. From the individual persons point of view, this is a good thing since they then can not be infected. From the point of view of the population, the more herd immunity that can be established, the slower the virus spreads, the less chance there is for a deadly form to spread.

I think it is a reasonable hypothesis that the very measures we are taking to slow the spread of the virus are keeping it weak (in terms of its lethality) and that we should continue. The main danger is if we stop the various containment methods. A fast spreading virus, if it become lethal, can kill huge numbers but it can only become fast spreading if we let down our guard. We must also treat any death from the virus as if the virus has become deadly and take extraordinary measures to isolate all contacts of the deceased person. It is counterproductive to hypothesize that the person was particularly vulnerable and that was the cause of death. In this case better to err on the side of caution.

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