Preparations for this year’s influenza season were upended Tuesday when federal officials announced that the country’s supply of flu vaccine would be half of what had been expected. Clinics offering the flu shot were canceled; the few that continued to operate were mobbed.

The problem lies with one of the two vaccine manufacturers. There were contamination problems at the manufacturer’s Liverpool plant. When British authorities suspended operations there, U.S. health officials began scrambling to ensure that the people at highest risk get the flu shots available.

Niranjan Bhat, medical epidemiologist in the influenza branch at the federal Centers for Disease Control and Prevention, answered questions about the problem and about the coming flu season.

Q: How many Americans are in the high-risk groups (including children younger than 2 and adults 65 and older; individuals with chronic illnesses such as asthma, diabetes and heart disease; and pregnant women) that are targeted for the 55 million flu-shot doses the country expects to receive?

A: An estimated 94.9 million people are in those high-risk groups.

Q: Based on past response rates, how much of that supply will be used for the high-risk groups?

A: It’s impossible to speculate how many of those included in the high-risk groups will get vaccinated this year. However, based on past years, only about half of those who are recommended to get vaccinated actually do.

Q:. Are there any other companies producing a flu vaccine for countries elsewhere in the world, and if so, why can the United States not secure doses from them?

A: There are about a dozen (to) 15 influenza vaccine manufacturers worldwide. The Food and Drug Administration regulates vaccines in the U.S. and would likely be the best source for information about how, or if, doses could be secured and used in the United States.

Q: If production were started immediately, when might additional vaccine doses be available?

A: Vaccine production is a lengthy process. By the time that vaccine would be manufactured and available, it would be very unlikely to help protect people during this flu season.

Q: How bad is this coming flu season predicted to be, especially compared with the past one, which hit earlier and with more intensity than anticipated?

A: It is not possible to accurately predict the features of any influenza season before the season begins. Although epidemics of flu happen in most years, the beginning, severity and length of the flu season can vary widely from year to year. Unfortunately, flu seasons are very unpredictable.

Q: What can you say about the strains of influenza expected this winter?

A: Influenza viruses are constantly changing, so it is not unusual for new strains of influenza to emerge at any time of the year. As of mid-September, both influenza A and B viruses were detected in the United States.

The influenza strains contained in the year’s vaccine (are) a good match for the viruses that have been detected.