Flu vaccines work by generating antibodies that recognize the surface of various types of influenza viruses. As a person goes through life, they will be exposed to multiple strains of flu. Each year, more than one serotype can circulate in the community. As time goes on, a person who has natural contact with influenza viruses will develop an extensive library of memory antibodies. When exposed to a strain to which a person has immunity, there might be just a mild cold, or no symptoms at all.
Flu vaccines do not prevent a person from getting sick in the winter time. There are over 200 serologically separate respiratory viruses that people encounter on a routine basis. Flu vaccine will not prevent rhinoviruses, adenoviruses, coronoviruses, nor the common 'vomiting' viruses like rota and norwalk virus.
Some people think of "flu" is "stomach flu" with vomiting/diarrhea illness, or they think that any winter cold/cough was "flu". Some people can experience very mild forms of influenza. This is because the severity of illness is linked to how large an initial "innoculum" of virus a person was exposed to. For example, if someone on the other side of the room coughs, and a person inhales a few influenza particles, it is likely that his or her immune system can eradicate the virus without getting a fever or missing school. Mild forms of flu can be indistinguisable from a common cold.
However, influenza has an ability to travel to the lungs, and can cause viral pneumonias, especially in the elderly. Throughout history, there have been strains of flu which are much more pathogenic. The dreaded Spanish flu of 1917-1918 had a virulence factor that was able to cause severe pulmonary injury people in healthy young people. The reasons for the virulence of that flu are still being investigated.
Flu vaccines are especially helpful in infants and young children because it gives them some basic blocks to build upon. Each year's flu vaccine contain protein antigens found in two strains of influenza A and one strain of influenza B. Which strains are included is determined by a panel of experts. Sometimes the strain that comes into the community doesn't match the vaccine strain. However, immunity against one strain can augment the response against other strains, making illness less severe.
Children have excellent immunological memory and even if the strain of flu present in the vaccine doesn't appear in a given year, the vaccine is not wasted. Some of the immune response cells will turn into memory T cells that can be mobilized upon exposure to that strain years later. A flu vaccine given to a toddler can help minimize illness to that strain when the child is in high school or college.
Some groups who can specially benefit from flu vaccine:
Infants (6 months +) In the first few months, placental antibodies can help the infant, but then the baby has to build up their own. Besides experiencing their first exposure to illnesses, people tend to hold them close and pass them around a room. Adults and older kids who are not 'sick' can unknowingly pass a virus through hand contact, or kissing the baby.
They get exposed to an enormous number of respiratory viruses and aren't inhibited about coughing into each others' faces. When influenza comes into a community, nursery school children are usually the first ones to get it. Also, it lasts in their nasal secretions for a longer period of time.
They often get "run down" during the week and party on weekends. The flu season often coincides with exams, or the return home for the holidays
Parents of Preschooler:
Nursery school children will bring lots of germs home. When they start running a fever, they often climb into their parents' bed and cough on them all night. This gives the parents a much larger viral "innoculum" than if they had gotten exposed to flu at an office party shaking someone's hands. If the parents are also sleep deprived, or stressed at work, they're more likely to get more of an illness.
Elderly people: especially those exposed to young children.
It is thought that a person's childhood-derived flu antibodies start to "fade" as they get into their 60's, 70's. Also, many older people have associated medical problems that make them more susceptible to pneumonia from flu.
Typical Family Scenario of influenza. :
The three year old comes home from school with a new strain of flu. He was fine in the morning, but spiked a 103 fever out of the blue in the afternoon. As the week goes on, the grammar school child (who has memory antibodies against that strain), just gets a 'sore throat' and one day of low grade fever . The eight month old also spikes a 103 fever and a croupy cough followed by an ear infection a few days later. The sleep deprived mother gets a bad bronchitis (she had inhaled a large dose of flu virions from the coughing preschooler). The father, who goes on a business trip a few days later, feels flu like symptoms and then a sinusitis (a combination of sleep deprivation, stress, airplane travel). The family then goes to visit the grandparents, who because of age, have lost some of the memory cells towards flu. The preschooler coughs directly into grandpa's face and he develops a viral pneumonia.
by E.Pytlak, M.D. Brooklyn, NY 11/06
Flu Shot Clinics