Flu Vaccine Effectiveness and Production Time

Every year, health authorities and flu vaccine manufacturers work together to make an influenza vaccine available to millions of people. In some years, the vaccine is more effective than in others; at worst, the seasonal flu shot may be virtually useless.

Meanwhile, health authorities are eyeing bird flu (avian influenza A/H5N1) as a possible future pandemic, yet a vaccine doesn't appear to be round the corner. And the 2009 swine flu (S-OIV A/H1N1) pandemic caught the world off-guard, and there are no timely prospects for a swine flu vaccine. Why can't health authorities reliably produce an effective influenza vaccine quickly?

The problem starts with the rapid mutation rate of the influenza virus, which causes its antigens to change.

An antigen is anything that provokes an immune system response. Flu virions are covered with proteins, which help the virus infect cells but which also act as antigens. Once "trained" to a specific antigen, the adaptive immune system is able to prevent that antigen from taking hold in the body again. This process is why a person becomes immune to most infectious diseases if they previously contracted the disease. It's also how vaccines work. A vaccine contains the antigens from a virus or bacteria, training the immune system without the need for actual infection.

With an average of over one mutation per replication cycle, both major types of flu — Influenza A and Influenza B — change rapidly in a process called antigenic drift. (Antigenic drift is different from antigenic shift, a special case of gene reassortment in Influenza A.) The antigens on the virus surface change, and the immune system fails to recognize the new antigens.

Influenza mutates so rapidly that each year, the seasonal flu consists of all-new strains. Previous flu shots are ineffective against these new strains, and a new influenza immunization tailored to the season's flu strains is necessary to prevent infection. Bird flu vaccines cannot be manufactured in advance because the specific flu strain does not even exist yet, and there is no way to predict which antigens it will have.

Each year, health monitoring authorities must race against the clock to predict the most likely strains for that year's influenza immunization. Scientists base their prediction on epidemiological studies of the virus in other parts of the world: the types of strains present and how they are evolving. They choose two strains of Influenza A and one of Influenza B. Predicting the flu is like predicting the weather: sometimes it is very accurate, but other times it misses the mark.

Why can't epidemiologists wait until the flu season is about to start to decide which strains will go into that year's flu vaccine? Flu vaccine manufacturers need a certain amount of time to manufacture the vaccine in sufficient quantities, so the flu strain prediction must be made months in advance. This lag time is also the reason why a swine flu vaccine could not be quickly made available during the 2009 swine flu outbreak.

The vaccine manufacturing process starts by growing large amounts of the flu virus in chicken eggs. The virus is then purified and killed with a toxic chemical. The killed virus is not able to cause infections, but still has the antigens that the immune system needs in order to provide immunity.

CDC Website (http://www.cdc.gov)