Rapid evolution of influenza RNA virus has resulted in limitation of vaccine effectiveness, increased emergence
of drug-resistant viruses and occurrence of pandemics. A new effective antiviral is therefore
needed for control of the highly mutative influenza virus. Teas prepared by the infusion method were
tested for their anti-influenza activity against clinical influenza A (H1N1) isolates by a 19-h influenza
growth inhibition assay with ST6Gal I-expressing MDCK cells (AX4 cells) using fluorogenic quantification
and chromogenic visualization. Guava tea markedly inhibited the growth of A/Narita/1/2009 (amantadine-
resistant pandemic 2009 strain) at an IC50 of 0.05% and the growth of A/Yamaguchi/20/06 (sensitive
strain) and A/Kitakyushu/10/06 (oseltamivir-resistant strain) at similar IC50 values ranging from 0.24% to
0.42% in AX4 cells, being 3.4- to 5.4-fold more potent than green tea (IC50 values: 0.27% for the 2009 pandemic
strain and 0.91% to 1.44% for the seasonal strains). In contrast to both teas, oseltamivir carboxylate
(OC) demonstrated high potency against the growth of A/Narita/1/09 (IC50 of 3.83 nM) and A/Yamaguchi/
20/06 (IC50 of 11.57 nM) but not against that of A/Kitakyushu/10/06 bearing a His274-to-Tyr substitution
(IC50 of 15.97 lM). Immunofluorescence analysis under a confocal microscope indicated that both teas
inhibited the most susceptible A/Narita/1/2009 virus at the initial stage of virus infection. This is consistent
with results of direct inhibition assays showing that both teas inhibited viral hemagglutination at
concentrations comparable to their growth inhibition concentrations but inhibited sialidase activity at
about 8-times higher concentrations. Guava tea shows promise to be efficacious for control of epidemic
and pandemic influenza viruses including oseltamivir-resistant strains, and its broad target blockage
makes it less likely to lead to emergence of viral resistance.