Instead, it contains pieces of the virus’s code that fool the immune system into producing antibodies for an infection that doesn’t exist. A week to 10 days later, the immune system is fully armed to do battle with a real flu virus.
“The usual reaction that people have is soreness in your arm,” Blackman said. In rare cases, according to the CDC, patients develop mild fever or headache.
A newer vaccine, intranasal — a nose spray — does contain live virus, she said. But you can’t get the flu from that vaccine either. Why? Because the virus has been altered to die at body temperature. It lives long enough to provoke an immune response, but it quickly dies once it’s in the body, Blackman said.
In exceptionally rare cases, the injectable vaccine can produce a very dangerous condition called anaphylactic shock, usually because a patient is allergic to chicken eggs, which are used in making the vaccine.
Most of those people have never received the vaccine before, Blackman said.
Building a vaccine
The flu season in the Northern Hemisphere runs from September to the end of March or sometimes into early April, depending on its progress, Blackman said. It’s the opposite in the Southern Hemisphere.
“Lots of people think that they get the flu because they got cold outside, but that’s generally not the case,” Blackman said. In fact, cold kills the virus. But cold weather forces people indoors, “and so it spreads more,” Blackman said.
A unit of the CDC decides which strains to focus on during the spring preceding the next season in September.
“They take a look at the pattern of the previous season right through the whole season and look for any new or different viruses that have been detected, different flu viruses, different strains,” Blackman said.