Most vaccines don't produce immunity until at least two weeks after they're given, so visit a doctor four to eight weeks before departure.

Avant's vaccine has produced some encouraging - if still preliminary - test results in people in the last year.

Also, the vaccine produces higher antibody that fights infection when given at this age compared to older ages.

Then physicians began to realize that for some children, the vaccine given at so young an age did not produce immunity.

The vaccine produced 100 percent immunity in the 6,000 women who received it as part of a multinational trial.

Manufacturers play a similar guessing game in deciding how much vaccine to produce.

Most vaccines don't produce immunity until around two weeks after they're given.

The vaccines produced for the Northern Hemisphere 2012-2013 season used:

This vaccine produced little immune memory, thus had a short duration of effect.

Existing vaccines or natural antibodies cannot produce an immune response to the new strain.