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A Timeline for infections and vaccinations. Groups of 4-5 mice were uninfected or infected i.n. with 1000 TCID 50 of A/California/07/2009 virus and then vaccinated i.m. 90 days later with either 1 g of A/Astrakhan/3212/2020 HA mRNA-LNP (mRNA) or 50 HAU of an inactivated A/Astrakhan/3212/2020 vaccine (inactivated vaccine). Sera were isolated 30 days after vaccination and IgG antibody titers were measured using ELISA plates coated with HA from A/California/07/2009 ( B ) and A/Astrakhan/3212/2020 ( D ). 50% Foci reduction neutralization test (FRNT 50 ) titers were determined using viruses expressing the HAs from A/California/07/2009 ( C ) and A/Astrakhan/3212/2020 ( E ). For the no vaccine/no prior exposure group, we tested serum obtained from mice before initiating the experiment. All data are shown as geometric means 95% confidence intervals. Data were compared using two-way ANOVA with Tukeys multiple comparisons test. Values were log-transformed before statistical analysis. Data are representative of 2 independent experiments. B Inactivated vaccine no prior exposure vs. no vaccine prior exposure * P = 0.0006; mRNA no prior exposure vs. no vaccine prior exposure * P = 0.0002; ( C ) Inactivated vaccine no prior exposure vs. no vaccine prior exposure * P = 0.0001; ( D ) Inactivated vaccine no prior exposure vs. no vaccine no prior exposure * P = 0.0444; all other comparisons * P < 0.0001. Red indicates mice with prior exposure. Source data are provided as a Source Data file.

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