Stabilized influenza hemagglutinin stem region trimers and uses thereof

A protein construct that comprises an HA protein domain and a linker sequence, wherein the HA protein domain comprises the sequence of an influenza hemagglutinin (HA) protein that lacks at least 95% of the head region amino acid sequence, and in place of the missing sequence comprises a linker sequence, wherein the linker sequence is less than 10 amino acids in length;

• and,
• wherein the HA protein domain comprises at least one alteration selected from the group consisting of:
• a. deletion of the amino acid region corresponding to amino acids N403-W435 of SEQ ID NO:8;
• and,
• b. substitution of at least one amino acid residue in a pair of amino acid residues in the HA protein domain, wherein the pair of amino acid residues form a non-covalent bond in the folded HA protein;
• and, wherein the strength of the non-covalent bond between the amino acid pair in the folded, protein construct is greater than the strength of the non-covalent bond between the amino acid pair in a folded, wild-type HA protein.