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The following are some main points of the observations, which the agency said "are based on limited data in only a few countries":
A NEW VIRUS TO WHICH PEOPLE HAVE LOW OR NO IMMUNITY
The H1N1 virus strain causing the current outbreaks is a new virus that has not been seen previously in either humans or animals. Although firm conclusions cannot be reached at present, scientists anticipate that pre-existing immunity to the virus will be low or non-existent, or largely confined to older population groups.
SECONDARY ATTACK RATE RANGES FROM 22 TO 23 PERCENT
H1N1 appears to be more contagious than seasonal influenza. The secondary attack rate of seasonal influenza ranges from 5 to 15 percent. Current estimates of the secondary attack rate of H1N1 range from 22 to 33 percent.
PEOPLE WITH UNDERLYING CONDITIONS MORE THREATENED
In terms of population vulnerability, the tendency of the H1N1 virus to cause more severe and lethal infections in people with underlying conditions is of particular concern. Outside Mexico, nearly all cases of illness, and all deaths, have been detected in people with underlying chronic conditions.
A YOUNGER AGE GROUP HAS BEEN AFFECTED
In the two largest and best documented outbreaks to date, in Mexico and the United States of America, a younger age group has been affected than seen during seasonal epidemics of influenza. Though cases have been confirmed in all age groups, from infants to the elderly, the youth of patients with severe or lethal infections is a striking feature of these early outbreaks.
SEVERITY OF DISEASE COULD CHANGE
Apart from the intrinsic mutability of influenza viruses, other factors could alter the severity of current disease patterns, though in completely unknowable ways, if the virus continues to spread.
Scientists are concerned about possible changes that could take place as the virus spreads to the southern hemisphere and encounters currently circulating human viruses as the normal influenza season in that hemisphere begins.