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Lung viral load. Viral RNA copies per milliliter in the lung suspension of mice at 72 h post infection in (A) homologous challenge two-dose immunization, (B) homologous challenge single-dose immunization, and (C) heterologous challenge groups. Abbreviations: HA, hemagglutinin; NP, nucleoprotein; HN, HA + NP; HNM, HA + NP + M2e. **p < 0.01 and ***p < 0.001.
