H1N1 Influenza A - the new swine flu variant - the incubation period and preventing spread of disease

Influenza A has an incubation period of about two days. Sometimes the incubation period can be as long as four days. During this period, a person is asymptomatic - that is has no symptoms of the flu. However, for the latter part of this period, a person may have sufficient amount of the virus to be able to transmit the virus to other people. This latter period is the time when a person is contagious (can transmit the virus to other people), even though he or she is asymptomatic. A person is usually contagious for about a day before symptoms develop.

The time during the incubation period when a person is asymptomatic (has no symptoms), but is contagious (can convey the virus to others), is one of the most important times because it is during this period that it is easy for someone to unknowingly spread the disease to others. Usually when a person becomes symptomatic, they become aware they have the flu and are more likely to be careful in taking steps to prevent transmission of the virus.

In the present circumstances, where we have a new virus, which could be a very serious pandemic virus, it is particularly important to take actions to reduce spread of the disease by people who are asymptomatic but are contagious.

We also know that we want to prevent rapid transmission to many people at the same time, because if this happens, the outbreak could quickly get out of control.

How do we address these issues?

In the instance of the present new H1N1 swine flu strain of influenza A, we know the major focus is Mexico. A simple measure is to ask those who have just returned from Mexico to avoid going to places where there are large groups of people for the first five days. Anyone who is asymptomatic for five days after returning from Mexico has virtually no chance of having swine flu picked up in Mexico, and can be considered in the clear.

It is also important that individuals who are in a situation where they might be incubating the swine flu that for five days after coming from Mexico they not visit individuals who are immune compromised unless major precautions are taken to prevent transmission of infection.

While others say the above measures are impractical, I say they are worth doing, particularly in the very early stages of the epidemic, until we know much more about the specifics of this particular virus.

I will add one more item. It is likely that this swine flu virus can be transmitted from people to pigs, though this has not been established with certainty at present. It may be that this particular H1N1 strain will cause respiratory disease in pigs, or it may be that it will not cause any disease in the pigs, but rather the pigs will carry the virus for a while without it affecting them at all - or causing any symptoms in them. For a whole variety of reasons it is smart to take extra measures to ensure the virus is not transmitted to pigs in Manitoba. While very stringent measures are already taken, one reasonable request is to ask anyone coming from a major focus of the epidemic (at this point Mexico), to not visit a hog barn in Manitoba for the first five days after they come to Manitoba. This is also a measure I have suggested.