Novartis has increased its offer to buy vaccines and biopharmaceutical company Chiron to $48 per share, in what seems to be an attempt to fend off the possibility of shareholders voting against the transaction at a meeting later this month.

The Swiss drugmaker had previously agreed to buy the 56% stake in Chiron it does not already own for $45 per share, or around $5.1 billion, but that offer did not find favour with some of the biotech’s shareholders.

Crucially, the two shareholders with sizeable chunks of Chiron’s equity who had had most vocally opposed the terms, CAM North America and ValueAct Capital, have come on board with the new offer. Chiron’s shareholders are now due to vote on the deal on April 19.

Novartis’ decision to firm up its bid comes shortly after two proxy advisory firms gave conflicting advice to shareholders on the transaction, one supporting Novartis’ offer and the other saying it undervalued Chiron, pointing to a recovery in the firm’s fortunes since it was forced to suspend manufacturing and sale of its Fluvirin flu vaccine in 2004, after contamination problems were uncovered at a UK manufacturing plant.

Meanwhile, Chiron also said it had settled claims in a previously reported stockholder action challenging the Novartis takeover, although the settlement is subject to court approval.

In a statement, Novartis said the raised bid “avoids the potential of value destruction for all Chiron shareholders that would have resulted from a failed transaction.”

The Swiss firm also said that a swift completion of the transaction will enable it to “make necessary investments required to quickly complete efforts aimed at fully restoring and increasing Chiron's production capabilities for seasonal influenza vaccines, as well as for vaccines against potential pandemic influenza strains such as the H5N1 virus.”