The Eurasian avian-like H1N1 (EAH1N1) swine flu viruses, which have circulated in pigs since 1979, have obtained the ability to infect humans and may "pose the highest pandemic threat" among the flu viruses currently circulating in animals, Chinese researchers said Monday.

"Pigs are considered important intermediate hosts for flu viruses," Chen Hualan, director of China's National Avian Influenza Reference Laboratory, who led the study, said in an written interview with Xinhua.

"Based on scientific analysis and comprehensive comparison of the main animal flu viruses: H1N1, H3N2, H5N1, H7N9, H9N2 and EAH1N1, we found the EAH1N1 is the one most likely to cause next human flu pandemic. We should attach great importance to the EAH1N1."

Two lineages of H1N1 swine influenza viruses (SIVs), classical H1N1 SIVs and EAH1N1 SIVs, have been circulating in pigs since 1918 and 1979, respectively. The classical H1N1 SIVs emerged in humans as a reassortant and caused the 2009 H1N1 flu pandemic.

The EAH1N1 SIVs have been detected in pigs in many Eurasian countries and have caused several human infections in European countries and also in China, where a fatal case was reported.

In the new study, Chen's team performed extensive flu surveillance in pigs in China and isolated 228 flu viruses from 36,417 pigs in slaughterhouses and on farms in 24 provinces from August 2010 to March 2013.

The researchers found that 139 of the 228 strains from pigs in 10 provinces in China belong to the EAH1N1 lineage. It indicated that "the EAH1N1 is the predominant swine flu virus circulating in pigs in China," Chen said.

After sequencing the genome of 40 representative EAH1N1 swine flu viruses that came from different farms, they divided the EAH1N1 into five genotypes, all of which have obtained the ability to infect humans.

"Most of the EAH1N1 swine flu viruses can spread efficiently among humans," Chen said. "Current human flu vaccines and the preexisting immunity in the human population can't offer enough protection against these viruses."

"Our study shows the potential of EAH1N1 SIVs to transmit efficiently in humans and suggests that immediate action is needed to prevent the efficient transmission of EAH1N1 SIVs to humans," the researchers wrote in their paper.

The findings were published in the US journal Proceedings of the National Academy of Science.