The new swine flu virus that broke out in Mexico is unique and potentially dangerous in one way. It is a virus that has a combination of gene segments from human, bird and swine viruses, and can potentially become infectious in humans that have no immunity to the new strain.

Influenza viruses can change its make-up in one of two ways: Antigenic drift is a series of mutations that cause the virus to gradually evolve over time. Antigenic shift is an abrupt change in the surface antigen proteins that suddenly creates a new subtype of the virus. In the history of influenza outbreaks, antigenic shift is the cause behind pandemics in 1918 (Spanish Flu), 1957 (Asian Flu) and 1968 (Hongkong Flu) because the populations have not developed antibody protection against the virus.

What's especially unique about this new swine flu strain is that it's a type A/ strain H1N1 and it hasn't been previously detected in pigs. Lab tests showed that the H1N1 is susceptible to the antiviral drugs oseltamivir (Tamiflu) and zanamivir and the US government is prepared to use the drugs to treat and prevent infection with swine influenza virus.

CDC Guidance in use of Tamiflu

WELLINGTON, New Zealand, April 27 NZPA - Dr Huang said from Geneva, where she is attending a WHO meeting on national influenza centre contingency plans, that it was "extremely important" to monitor oseltamivir resistance because of the reliance being placed on Tamiflu stockpiles in the event of a pandemic.

She warned there was a possibility that as the swine influenza spread among people already exposed to Tamiflu-resistant strains of influenza, the swine flu may evolve resistance to the drug.

"Influenza virus is notoriously unpredictable," she said.

"It is very important to monitor closely in order to provide early warning if (this) situation ... emerged."

New Zealand virologist Richard Webby, director of the WHO collaborating centre for studies on the ecology of influenza viruses in lower animals and birds at St Jude Children's Research Hospital in Memphis, Tennessee, told NZPA the risk of a Tamiflu-resistant swine flu evolving was a "valid possibility".

"Flu viruses do exchange gene segments and it is possible that the swine virus could swap out its neuraminidase (the target for oseltamivir) with a human strain," he said.

Problems would arise if the resulting virus was able to grow well and transmit between humans.

The data on the current flu season, 2008-2009 is obviously not complete since the season is not yet finished, but an analysis on that which has been gathered so far shows that Tamiflu resistance in H1N1 type A strains continue at a high level. Up to 19 February 2009, Tamiflu resistance was found in 264 of 268 (98.5 per cent) of the H1N1 type A viruses tested by the CDC.

The authors wrote that:

"The emergence of oseltamivir resistance has highlighted the need for the development of new antiviral drugs and rapid diagnostic tests that determine viral subtype or resistance, as well as improved representativeness and timeliness of national influenza surveillance for antiviral resistance."

In December last year the CDC issued draft guidelines for the use of antiviral flu medications in line with what they found after analyzing the data coming in for the current season. They recommended that doctors and other health professionals:

"Consider the results of patient testing and local influenza surveillance data on circulating types and subtypes of influenza viruses in deciding whether oseltamivir [tamiflu] alone could be used. These guidelines provide options, including preferential use of [the anti-viral drug] zanamivir or a combination of oseltamivir and [the anti-viral drug] rimantadine, which might be more appropriate in treating patients who might have influenza caused by an oseltamivir-resistant virus."