A Department of Homeland Security (DHS) report from 9/19/09 indicates that the period of infectivity of A-H1N1/09 is longer than originally believed, and longer than that of the seasonal flu.
Studies on how long a person remains contagious back up the previously reported CDC guidelines that one should be suspected to be capable of spreading the Novel Swine Flu for one full week after the symptoms start or until 24 hours after the fever subsides [while taking no fever reducers] which ever is longer, rather than the most recent CDC guidelines that waiting for 24 hours after fever subsides without taking fever reducers is long enough.
In fact, the best determination can probably be made by how long the cough lasts.
The studies show that in some cases the virus remains in the nasal passages of patients for up to 16 days after symptoms begin. Below are some excerpts from the report:
Swine flu also appears to be contagious longer than ordinary seasonal flu, several experts said.
When the coughing stops is probably a better sign of when a swine flu patient is no longer contagious, experts said after seeing new research that suggests the virus can still spread many days after a fever goes away.
Using a very sensitive test to detect virus in the nose or throat, [the study] found that 80 percent had it five days after symptoms began, and 40 percent seven days after. Some still harbored virus as long as 16 days later. How soon they started on antiviral medicines such as Tamiflu made a difference in how much virus was found, but not whether virus was present at all. . .
Doctors know that people can spread ordinary seasonal flu for a couple of days before and after symptoms start by studying virus that patients shed in mucus. The first such studies of swine flu are just coming out now, and they imply a longer contagious period for the novel bug.