Shareholders in Chiron are being offered diverging advice on whether the vaccine and biopharmaceutical company should accept a $5.1 billion takeover offer from Swiss drugmaker Novartis.

One proxy advisory firm – Institutional Shareholder Services – has advised against the deal, saying that Chiron’s business has improved since the offer was made, while another - Proxy Governance Inc – said Chiron’s shareholders should support it. They are due to vote on Novartis’ proposed takeover offer on April 12.

Novartis’ offer for the company came as Chiron was still mired in difficulties caused by manufacturing compliance issues at a flu vaccine plant in the UK, which forced it to suspend shipments of its widely-used Fluvirin product in October 2004, playing havoc with its revenues and driving it into the red. Since then, production and shipping of Fluvirin has restarted and the company returned to profit in the fourth quarter of 2005.

The contrary advice adds to the uncertainty over the outcome of the shareholder vote. Earlier this year, CAM North America, which holds 7.6% of Chiron, said it would vote against the transaction. It joined ValueAct Capital, which holds around 5% of the stock and said last December it would oppose the deal.

Chiron said in a statement that its independent directors unanimously recommend that shareholders vote for the deal because it would deliver ‘full and fair value’.