This is the VOA Special English Health Report.

Most vaccines are designed with the same goal in mind. That is, to help the body's own defense system prevent a disease by producing antibodies against it. Antibodies are disease-fighting proteins. The immune system produces them in reaction to viruses, bacteria and other invaders.

The vaccine tricks the body into thinking it has already successfully defeated the disease. To activate the immune system, vaccines commonly introduce the disease-causing virus or bacteria into the body. But they use weakened or killed versions.

Weakened viruses are used, for example, in vaccines against chickenpox, measles, mumps and rubella. To prevent polio, the Sabin vaccine uses a weakened form of the virus; the Salk vaccine uses a killed version.

Experts say vaccines that use killed or inactivated virus can be safely given even to people with damaged immune systems.

Researchers may spend years working on a vaccine. They have still not succeeded against, for example, H.I.V., the virus that causes AIDS, or against malaria, but they are trying.

And not all vaccines offer long-term protection. The tetanus vaccine is a good example. It offers protection for only about ten years. Then a person must be immunized again.

Some vaccines are made with animal material. For example, influenza vaccine is grown in chicken eggs. This can be a problem for people who are allergic to eggs. Also, the process is complex.

But things could change in the future. A new study in the Journal of the American Medical Association suggests that flu vaccine could come from insect cells.

Researchers in the United States tested a flu vaccine made from caterpillar cells. The study involved four hundred sixty people. There were two versions of the vaccine, one stronger than the other.

The people were not told whether they were getting the vaccine or a substitute, a placebo. Here is what the scientists reported: Seven people in the placebo group caught the flu. So did two people who received the lower strength vaccine. But no one in the stronger vaccine group got the flu.

Protein Sciences, a vaccine maker, paid for the study. The company plans to begin testing the experimental flu vaccine on a larger group in order to seek government approval.

And that's the VOA Special English Health Report, written by Caty Weaver. For more health news, along with transcripts and audio files of our reports, go to voaspecialenglish.com. I'm Steve Ember.