Many people are feeling a sense of deja-vu about this newest influenza outbreak.  Two years ago, public health authorities were on the alert for a possible outbreak of bird flu, based on the deaths of 257 individuals around the world (out of a confirmed 421 cases in 15 countries).  The possible spread of that strain was a justifiable source of concern for governments around the world.  Fortunately, the pandemic of bird flu did not materialize, and the number of infected individuals remained low:  most of the infections had come about as a result of bird to human contact.

This H1N1 strain of swine flu is quite different. Its emergence provides an opportunity to contrast three important features of the influenza virus:  transmissibility, infectivity and severity.

Transmissibility refers to the ease with which the virus can be passed from one host to another.  H1N1 has our attention precisely because of its apparent  transmissibility– or more precisely, because of its capacity for transmission from one human host to another.   This stands in stark contrast with the bird flu, where it appeared that that influenza strain was seldom, if ever, capable of moving from an infected human to an uninfected human host.  As a result, virtually all of the confirmed bird flu cases occurred in individuals that had been in recent contact with infected poultry.  (As a general rule, the rate of viral transmission from an animal reservoir to a human host is exceedingly low, and requires repeated contact with infected animals or birds).

The H1N1 strain likely originated through pig-human contact, although this remains to be confirmed.  The location of this original transfer is still unknown, and may or may not be in the small town of La Gloria, in the state of Veracruz,  Mexico.  What is clear is that the majority of confirmed cases of H1N1 around the globe cannot be traced to pig-human contacts: people are not getting this flu from contact with pigs (or pork).

Infectivity: Infectivity measures the ease with which individuals exposed to a particular viral strain develop the infection.  Infectivity and transmissibility are clearly linked, but we do not as yet know much about the the infectivity of H1N1 strain.  The fact that the overwhelming majority of our immune systems have never seen anything resembling the H1N1 strain before increases this strain’s infectivity (making it, from the virus’ point of view, a particularly successful strain); we are still unclear about the extent of exposure required  to contract H1N1.  (Not surprisingly, given our previous discussion, the infectivity of the the bird flu strain was exceedingly low.)

Severity:  The severity of the H1N1 strain, as opposed to that of the bird flu, presents a  startling contrast.  While the lethality of the bird flu was close to 57%, H1N1 appears to be far less lethal, and results in far milder symptoms.   Some confusion on this score surfaced in earlier reports which suggested a high mortality rate from the H1N1 virus in Mexican cases.  It now appears that the number of deaths provisionally attributed to H1N1 may have been overestimated, and has been revised down from 176 to 101.  More subtly, the number of total cases of H1N1 in Mexico (the denominator when you are calculating the mortality rate for H1N1), may have been significantly underestimated.  This underestimate  arises for a variety of reasons: lag times in reporting, a weaker infrastructure in Mexico for the diagnosing and collating of cases,  the challenges associated with confirming a “probable case” as an unambiguous H1N1 infection, and the likelihood that in rural areas many individuals with the infection will not or cannot not enter the health care system.

The possibility that H1N1 first emerged in Mexico as a more virulent strain, and has since evolved into a milder strain cannot be ruled out, nor can the hypothesis that the health status of most of the Mexican population (including the presence of previous chronic infections) increases the severity of symptoms associated with H1N1 infection.  What is absolutely clear is that cases of H1N1 outside of Mexico are so far presenting with symptoms no more severe than those seen with ordinary seasonal flu strains.