The new H1N1 influenza virus bears a disturbing resemblance to the virus strain that caused the 1918 flu pandemic, with a greater ability to infect the lungs than common seasonal flu viruses, researchers reported on Monday. Separately, a top official at the World Health Organization said Monday a fully licensed swine flu vaccine might not be available until the end of the year. The report could affect many countries' vaccination plans. But countries could use emergency provisions to get the vaccines out quicker if they decide their populations need them. The swine flu viruses currently being used to develop a vaccine aren't producing enough of the ingredient needed for the vaccine, and WHO has asked its laboratory network to produce a new set of viruses as soon as possible. Other tests showed the virus could be controlled by the antiviral drugs Relenza, made by GlaxoSmithKline, and Tamiflu, made by Roche AG, the researchers said. The World Health Organization said on Monday that vaccine makers should start making immunizations against H1N1 and that healthcare workers should be first in line to get them. The WHO has previously estimated that the world could have as many as 4.9 billion doses of H1N1 swine flu vaccine ready for the next flu season - but this assumes people only need one shot and production yields are similar to seasonal vaccine.