The first genetic analysis of the efficacy of swine flu virus H1N1 transmission from person to person revealed that it spreads barely well enough to keep itself going.
The analysis also suggested that the virus might have started circulating as long ago as January.
However, because of the scarcity of cases to analyse, the calculation is still uncertain, as many believe that the circulation could have started more recently, or as far back as September.
Nicholas Grassly of Imperial College London and Andrew Rambaut of the University of Edinburgh, UK, have analysed the rate of spread.
Their analysis is based on the small mutations that have accumulated in almost two-dozen genetic sequences produced so far, from viruses collected from patients in Mexico and the US.
Unlike H5N1 bird flu, all the genetic sequences of this H1N1 are being posted on bulletin boards like GISAID, so that scientists can access them and compare preliminary analyses.
Scientists who protested that H5N1 sequences were not being made freely available set up the GISAID system in 2006.
"The limited sampling so far gives rise to considerable uncertainty in the estimate," New Scientist quoted Rambaut as saying.
However, if the rate at which genes mutate is similar for this virus as for other H1N1 viruses, the number of mutations that have accumulated so far have indicated that it has been circulating since January, or even September 2008.
If the new virus spreads from one infected person to the next at about the same speed as ordinary flu, it could give an idea of how many cases there may have been in that time.
A mathematical model permits the calculation of an important variable called R0, the number of additional people infected, on average, by each case.
If R0 is less than one, an infection dies out.
Also, Grassly cautioned that the estimate is very preliminary.
However, with newly available data, he gets an R0 of 1.16, enough for the virus to keep going, but only just.
This comes as good news, as epidemiological theory suggests that the lower the R0, the easier it may be to snuff the virus out by further hindering its spread.
And now the onus lies on how quickly the new H1N1 virus from swine adapts to people.