On a late Thursday conference call,

MedImmune

(MEDI)

executives expressed confidence about the eventual approval of the company's experimental flu vaccine, FluMist, but they did not alter their previously cautious stance about whether the vaccine will be ready for the upcoming 2002-03 flu season.

This morning, the biotech firm said it

received a "complete response letter" from the Food and Drug Administration, which requested additional information on FluMist before the regulatory agency could render a decision on the vaccine's approval.

Speaking on the conference call, MedImmune CEO David Mott said the company believes it can respond to the FDA's request for more FluMist information within 30 to 45 days. But he said it was too early to determine whether that would give the agency enough time to render a decision by the beginning of November, which is typically the start of flu immunization season. MedImmune received the complete review letter Wednesday afternoon and has not yet talked to FDA officials about the contents.

In fact, Mott seemed to hint that such a speedy FDA turnaround would take a Herculean effort.

"The timeline is tight," he said, when asked about the chances of a 2002 approval. He added that getting approval this year would require the FDA to work much on a much faster regulatory timeline than is customary in these situations.

In the past, Mott has given the company a 50-50 chance of getting FluMist launched this year. When asked to update that prediction Thursday, he declined, saying that the vaccine, which is inhaled instead of injected, still had a "potential" for a 2002 launch.

For long-term MedImmune investors, making a distinction between a 2002 and 2003 FluMist approval probably seems like hair-splitting. But if the vaccine isn't ready this year, the company will have to cut its 2002 sales and earnings estimates. MedImmune is expected to earn 63 cents per share this year, which includes profits from 2002 FluMist sales and royalty payments from partner

Wyeth

(WYE)

.

Salomon Smith Barney biotech analyst Elise Wang believes a delayed FluMist launch to 2003 likely will cost MedImmune about 25 cents per share in 2002 earnings. She rates the stock outperform and doesn't do banking for the company.

But there's more at risk to MedImmune than just the top line. The company is going ahead with plans to produce 4 million doses of FluMist vaccine for this year, just in case approval comes. The makeup of flu vaccines, however, can change year to year, depending on prevalent strains of the disease, so this stockpile may have to be written off if the company can't sell it this year. This potential writedown could exacerbate the company's 2002 losses.

Shares of MedImmune were among the top performers in the biotech sector Thursday, as investors interpreted the vague comments issued this morning by the company as a positive sign. Investors bid up MedImmune $3.36, or 16%, to $24.46 per share during the regular trading session.

More than 25 million shares traded hands, or more than 5 times average volume. Some of that was surely short covering since MedImmune has come under heavy selling pressure in anticipation of the FDA decision.

Mott said the FDA's complete response letter included 68 additional questions on FluMist -- 24 related to the manufacturing process and 64 on the vaccine's clinical characteristics. Mott said most of the questions were relatively minor, although some were weighty in nature. One issue of concern for the FDA is the potential for elevated rates of asthma and wheezing in kids age 18 to 35 months old. This potential safety risk was previously discussed at an FDA advisory committee meeting in July 2001.