Dec 12 (Reuters) - Drugmaker AstraZeneca on Tuesday agreed to buy vaccine developer Icosavax in a deal valued at up to $1.1 billion to bolster its respiratory syncytial virus (RSV) vaccine portfolio.

The deal values Icosavax at $15 per share in cash at closing, plus a non-tradable contingent value right for up to $5 per share in cash if certain milestones are met.

The upfront payment of $15 apiece represents a premium of about 43% to U.S.-listed Icosavax's last close.

(Reporting by Yadarisa Shabong in Bengaluru; Editing by Sonia Cheema)