mRNA vaccines against H10N8 and H7N9 influenza viruses of pandemic potential are immunogenic and well tolerated in healthy adults in phase 1 randomized clinical trials

Two double-blind, placebo-controlled, randomized phase 1 clinical trials recruited participants from single centers in Germany (H10N8) and the United States (H7N9) from December 2015 to August 2017. Participation of healthy adults (18 to 64 years old for study H10N8; 18 to 49 years old for study H7N9). Participants received the vaccine or placebo in a series of 2 doses 3 weeks apart. H10N8 intramuscular (IM) dose levels of 25, 50, 75, 100, and 400 µg and intradermal dose levels of 25 and 50 µg were evaluated. The dose levels of H7N9 IM 10, 25, and 50 µg were evaluated; a 2-dose series separated by 6 months was also evaluated. The primary endpoints are safety (adverse events) and tolerability. Secondary immunogenicity results include humoral reactions (hemagglutination inhibition [HAI], microneutralization [MN]) and cell-mediated reactions.

The H10N8 and H7N9 mRNA IM vaccines showed good safety and reactogenic characteristics. No serious adverse events related to the vaccine were reported. For H10N8 (N = 201), an IM dose of 100 μg induced an HAI titer ≥ 1:40 in 100% of the participants and a MN titer ≥ 1:20 in 87.0% of the participants. The 25 μg intradermal injection dose induced an HAI titer> 1:40 in 64.7% of the participants, while 34.5% of the participants received the IM dose. For H7N9 (N = 156), intramuscular doses of 10, 25, and 50 µg achieved HAI titer ≥ 1:40 in 36.0%, 96.3%, and 89.7% of participants,
respectively. MN titer ≥ 1:20 reached 100% in the 10 and 25 µg groups, and 96.6% in the 50 µg group. The seroconversion rate of H10N8 (100 µg IM) was 78.3% (HAI) and 87.0% (MN), and the seroconversion rate of H7N9 (50 µg) was 96.3% (HAI) and 100% (MN). No significant cell-mediated response was detected in any study.

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