Updated from 6:30 p.m. EDT

The Food and Drug Administration on Wednesday said it had released a small amount of flu vaccine made by Chiron ( CHIR), making the company the fourth vaccine maker to enter this year's flu market.

Chiron canceled all of its U.S. flu vaccine last year after being cited by health regulators for manufacturing problems at its plant in Liverpool, England. Last year, Chiron had been expected to provide 48 million to 50 million doses.

This year, the Emeryville, Calif., biotechnology company has scaled down its estimates to 18 million to 26 million doses.

A Chiron spokeswoman said Wednesday it would take "a matter of days" to send the finished product to distributors as the company "finalizes its internal release" process.

The FDA said it approved three bulk lots of the Chiron vaccine known as Fluvirin. The Chiron spokeswoman estimated that those lots would yield about 1.5 million doses of vaccine.

Chiron got a late start in getting its vaccine ready for the market because of the many requirements from both British and U.S. regulators.

The FDA noted that Sanofi-Aventis ( SNY) has been cleared to sell 65 lots, GlaxoSmithKline ( GSK) has been approved to sell 21 lots, and MedImmune ( MEDI) has been cleared to sell 14 lots.

Lot sizes can vary among producers, according to a recent research report from Sanford C. Bernstein, which estimates that lots can yield 250,000 to 500,000 doses.

The FDA has been approving lots from Chiron's competitors since early September. During the current flu season, Sanofi-Aventis is expected to provide 60 million doses of its Fluzone. GlaxoSmithKline is in line to offer 8 million doses of its Fluarix, and MedImmune is expected to provide 3 million doses of its nasal spray vaccine FluMist.

Chiron's stock closed Wednesday at $43.04, down 6 cents.