In the case of influenza, effectiveness is expected to be lower than the efficacy because it is measured using the rates of influenza-like illness, which is not always caused by influenza. Studies on the effectiveness of flu vaccines in the real world are difficult; vaccines may be imperfectly matched, virus prevalence varies widely between years, and influenza is often confused with other influenza-like illnesses. However, in most years (16 of the 19 years before 2007), the flu vaccine strains have been a good match for the circulating strains, and even a mismatched vaccine can often provide cross-protection. The virus rapidly changes due to antigenic drift, a slight mutation in the virus that causes a new strain to arise.The effectiveness of seasonal flu vaccines varies significantly, with an estimated average efficacy of 50–60% against symptomatic disease, depending on vaccine strain, age, prior immunity, and immune function, so vaccinated people can still contract influenza. The effectiveness of flu vaccines is considered to be suboptimal, particularly among the elderly, but vaccination is still beneficial in reducing the mortality rate and hospitalization rate due to influenza as well as duration of hospitalization. Vaccination of school-age children has shown to provide indirect protection for other age groups. LAIVs are recommended for children based on superior efficacy, especially for children under 6, and greater immunity against non-vaccine strains when compared to inactivated vaccines.From 2012 to 2015 in New Zealand, vaccine effectiveness against admission to an intensive care unit was 82%.
As of November 2007, both the conventional injection and the nasal spray are manufactured using chicken eggs. The European Union also approved Optaflu, a vaccine produced by Novartis using vats of animal cells. This technique is expected to be more scalable and avoid problems with eggs, such as allergic reactions and incompatibility with strains that affect avians like chickens.Influenza vaccines are produced in pathogen-free eggs that are eleven or twelve days old. The top of the egg is disinfected by wiping it with alcohol and then the egg is candled to identify a non-veinous area in the allantoic cavity where a small hole is poked to serve as a pressure release. A second hole is made at the top of the egg, where the influenza virus is injected in the allantoic cavity, past the chorioallantoic membrane. The two holes are then sealed with melted paraffin and the inoculated eggs are incubated for 48 hours at 37 degrees Celsius. During incubation time, the virus replicates and newly replicated viruses are released into the allantoic fluidAfter the 48-hour incubation period, the top of the egg is cracked and the ten milliliters of allantoic fluid is removed, from which about fifteen micrograms of the flu vaccine can be obtained. At this point, the viruses have been weakened or killed and the viral antigen is purified and placed inside vials, syringes, or nasal sprayers. Done on a large-scale, this method is used to produce the flu vaccine for the human population.