The H1N1 pandemic strain, or swine flu, is resistant to the two older drugs.
They say the seasonal vaccine will not protect against the pandemic strain.
The chances of spotting a pandemic strain early enough to contain it are slim.
And the 2009 pandemic strain, an A(H1N1), is predominant in Southeast Asia and in Mexico.
There is an H1N1 human strain in this year's flu shot, and all H1N1 flus are descendants of the 1918 pandemic strain.
The vaccines produced every year to prevent seasonal flu are unlikely to be of any use in warding off a pandemic strain.
It is also unclear whether a vaccine currently under production would have any effect against an emergent pandemic strain, even its manufacturer, Sanofi Pasteur, admits.
The manufacturing deficiencies could portend catastrophe should a much-feared pandemic strain of influenza emerge and sweep across a world that would be largely unprotected.
There has never been a case of avian flu reported in the United States, and it has not mutated into a pandemic strain.
There is an H1N1 human strain in this year's shot, and all H1N1 flus are descendants of the 1918 pandemic strain.
Fittingly, the 1918 pandemic strain has been designated as H1N1.
