11.   Aventis says it fell behind in making the vaccine after routine maintenance at its factories took longer than expected.

12.   Because Acambis already is making smallpox vaccine, it was able to promise it faster and for less money than its rivals.

13.   Because the flu vaccine is made from viruses grown in eggs, however, you should not get the vaccine if you are allergic to eggs.

14.   A company in Lansing, Mich., BioPort Corp., makes a vaccine for humans, and many military personnel are required to be vaccinated.

15.   Acambis is the only company in the world licensed to make the vaccine.

16.   But experts say the shortages are sobering, underscoring the shrinking number of companies making vaccines and a tangle of legal and financial issues that may be choking supplies.

17.   But in the past, few of the major pharmaceutical companies have been interested in making vaccines for the government.

18.   But it will take a year or more to replace existing supplies with reformulated products, because it can take many months to grow cultures used in making vaccines.

19.   But now Aventis Pasteur is the only company making the vaccine, and earlier this year that caused troublesome shortages in some areas.

20.   But that was before the Air Force sergeant at Whiteman Air Force Base learned that cells derived from aborted fetuses were used to make the vaccine.