* Vaccine alliance with Shenzhen Neptunus* Glaxo to take 40 pct...

* Vaccine alliance with Shenzhen Neptunus
* Glaxo to take 40 pct initially, majority within 2 years

* JV to make flu vaccines for China, Hong Kong and Macau

* Deal bolsters Glaxo position in key emerging market (Adds analyst comment, background)

By Ben Hirschler

LONDON, June 9 (Reuters) - GlaxoSmithKline (GSK.L), the world's second biggest drugmaker, has forged a new alliance with Shenzhen Neptunus to make flu vaccines for the Chinese market, it said on Tuesday.

Glaxo will take an initial 40 percent stake in the joint venture for a contribution of cash and assets equivalent to 21 million pounds ($34 million), and will buy additional shares to obtain majority control within the next two years.

The British-based drugmaker is already one of the world's leading producers of flu vaccines and the deal secures it a position in the important Chinese market at a time when demand for flu immunisation is soaring.

WestLB analyst Simon Mather said the joint venture would enable Glaxo to leverage its adjuvant, or vaccine additive, technology, which makes vaccines more effective, and also wins the company further access to emerging markets.

Over the next few years, the new alliance will supply seasonal, pre-pandemic and pandemic flu vaccine for China, Hong Kong and Macau.

"This deal is a further demonstration of CEO Andrew Witty's desire to increase exposure in the emerging markets ... at a time where the threat of a global swine flu pandemic remains real," Mather said.

Glaxo, like other big flu vaccine manufacturers, such as Sanofi-Aventis (SASY.PA) and Novartis (NOVN.VX), has seen extra demand for vaccine this year to deal with the spread of the H1N1 strain of flu, which is now on the verge of a pandemic.

WestLB currently forecast 2009 vaccines sales for Glaxo of 3.1 billion pounds, up 23 percent on 2008, and as much as 3.7 billion if the company can supoply recently announced European government orders for pandemic vaccine by the end of the year. (Editing by Mike Nesbit and Dan Lalor) ($1 = 0.6230 pounds)