The 2009 swine flu pandemic vaccines were influenza vaccines developed to protect against the pandemic H1N1/09 virus. These vaccines either contained inactivated (killed) influenza virus, or weakened live virus that could not cause influenza. The killed virus was injected, while the live virus was given as a nasal spray. Both these types of vaccine were produced by growing the virus in chicken eggs. Around three billion doses were produced, with delivery in November 2009.
In studies, the vaccine appeared both effective and safe, providing a strong protective immune response and having a similar safety profile to the usual seasonal influenza vaccine. However, about 30% of people already had some immunity to the virus, with the vaccine conferring greatest benefit on young people, since many older people are already immune through exposure to similar viruses in the past. The vaccine also provided some cross-protection against the 1918 flu pandemic strain.
Early results (pre-25 December 2009) from an observational cohort of 248,000 individuals in Scotland showed the vaccine to be effective at preventing H1N1 influenza (95.0% effectiveness [95% confidence intervals 76.0–100.0%]) and influenza-related hospital admissions (64.7% [95% confidence intervals 12.0–85.8%]).
Developing, testing, and manufacturing sufficient quantities of a vaccine is a process that takes many months. According to Keiji Fukuda of the World Health Organization, "There's much greater vaccine capacity than there was a few years ago, but there is not enough vaccine capacity to instantly make vaccines for the entire world's population for influenza." The nasal mist version of the vaccine started shipping on 1 October 2009.
Types of vaccine
Two types of influenza vaccines were available: TIV (flu shot (injection) of trivalent (three strains; usually A/H1N1, A/H3N2, and B) inactivated (killed) vaccine) or LAIV (nasal spray (mist) of live attenuated influenza vaccine.)
TIV works by putting into the bloodstream those parts of three strains of flu virus that the body uses to create antibodies; while LAIV works by inoculating the body with those same three strains, but in a modified form that cannot cause illness.
LAIV is not recommended for individuals under age 2 or over age 49, but might be comparatively more effective among children over age two.
Manufacturing methods
For the inactivated vaccines, the virus is grown by injecting it, along with some antibiotics, into fertilized chicken eggs. About one to two eggs are needed to make each dose of vaccine. The virus replicates within the allantois of the embryo, which is the equivalent of the placenta in mammals. The fluid in this structure is removed and the virus purified from this fluid by methods such as filtration or centrifugation. The purified viruses are then inactivated ("killed") with a small amount of a disinfectant. The inactivated virus is treated with detergent to break up the virus into particles, and the broken capsule segments and released proteins are concentrated by centrifugation. The final preparation is suspended in sterile phosphate buffered saline ready for injection. This vaccine mainly contains the killed virus but might also contain tiny amounts of egg protein and the antibiotics, disinfectant and detergent used in the manufacturing process. In multi-dose versions of the vaccine, the preservative thimerosal is added to prevent growth of bacteria. In some versions of the vaccine used in Europe and Canada, such as Arepanrix and Fluad, an adjuvant is also added, this contains squalene, vitamin E and an emulsifier called polysorbate 80.
To make the live vaccine, the virus is first adapted to grow at and then grown at this temperature until it loses the ability to cause illness in humans, which requires the virus to grow at normal human body temperature of . Multiple mutations are needed for the virus to grow at cold temperatures, so this process is effectively irreversible and once the virus has lost virulence (become "attenuated"), it will not regain the ability to infect people. The attenuated virus is then grown in chicken eggs as before. The virus-containing fluid is harvested and the virus purified by filtration; this step also removes any contaminating bacteria. The filtered preparation is then diluted into a solution that stabilizes the virus. This solution contains monosodium glutamate, potassium phosphate, gelatin, the antibiotic gentamicin, and sugar.