Why would two Swine Flu shots be needed?

Answer 1

The vaccines approved for use in the US will only require a single dose for most people to develop immunity to H1N1/09 Swine Flu. It was originally believed that two might have been needed because it is a totally new strain of influenza that has never existed before. That means that would take a strong dose for our bodies to begin to react fully to the virus and make the appropriate anti-viral antibodies. Our immune systems have to be exposed to enough of the antigen/virus to know how to make the right types of cells that can neutralize the antigen by binding specifically to it. However, the human trials proved that a single dose of the vaccine made with the particularly good seed virus that was selected for use in developing the vaccines was sufficient.

For the seasonal flu shots we have known that we would only need to get one because our bodies already have been exposed to some very similar strains of the seasonal flu either through our genes or our mother's milk and by weakened or inert particles of the virus in the environment. That means that it only takes one small dose to trigger our immune systems to create the right antibodies to fight the particular viral strains and create full immunity.

With the H1N1/09 vaccine, infants over 6 months old and children under 10 who have immature immune systems that have not been exposed to as many virus strains in the environment as adults have, will still need to have two vaccinations spread a month apart for them to be able to have the proper immune response that will result in full protection.

(Infants under 6 months old can not be vaccinated at all.)