Vaccine efficacy

Vaccine efficacy is the percentage reduction of disease in a vaccinated group of people compared to an unvaccinated group, using the most favorable conditions. Vaccine efficacy was designed and calculated by Greenwood and Yule in 1915 for the cholera and typhoid vaccines. It is best measured using double-blind, randomized, clinical controlled trials, such that it is studied under “best case scenarios.” [1] Vaccine effectiveness differs from vaccine efficacy in that vaccine effectiveness shows how well a vaccine works when they are always used and in a bigger population whereas vaccine efficacy shows how well a vaccine works in certain, often controlled, conditions. Vaccine efficacy studies are used to measure several possible outcomes such as disease attack rates, hospitalizations, medical visits, and costs.

Vaccine efficacy formula

The outcome data (vaccine efficacy) generally are expressed as a proportionate reduction in disease attack rate (AR) between the unvaccinated (ARU) and vaccinated (ARV) studies can be calculated from the relative risk (RR) of disease among the vaccinated group with use of the following formulas.

The basic formula[2] is written as:


  • = Vaccine efficacy,
  • = Attack rate of unvaccinated people,
  • = Attack rate of vaccinated people.

An alternative, equivalent formulation of vaccine efficacy

where is the relative risk of developing the disease for vaccinated people compared to unvaccinated people.

How it's tested

Vaccine efficacy differs from vaccine effectiveness in the same way that an explanatory clinical trial differs from an intention to treat trial: vaccine efficacy shows how effective the vaccine could be given ideal circumstances and 100% vaccine uptake; vaccine effectiveness measures how well a vaccine performs when it is used in routine circumstances in the community.[3] Since vaccine efficacy is based on a population that are placed in certain controlled environment, this study becomes more effective. If the criteria changed, such as if it was based on a larger population that wasn’t as restricted and in a more natural environment, that would be the vaccines effectiveness. What makes the vaccine efficacy applicable is that it also shows the disease attack rates as well as a tracking of vaccination status. Vaccine effectiveness is a lot more easily tracked than the vaccine efficacy considering the difference in environment; however, the vaccine efficacy is more expensive and very difficult to conduct. Because the trial is based on people who are taking the vaccination and people who aren’t, there is a risk for disease, and optimal treatment is needed for those who become infected. The advantages of a vaccine efficacy have control for all biases that would be found with randomization, as well as prospective, active monitoring for disease attack rates, and careful tracking of vaccination status for a study population there is normally a subset as well, laboratory confirmation of the infectious outcome of interest and a sampling of vaccine immunogenicity. The major disadvantages of vaccine efficacy trials are the complexity and expense of performing them, especially for relatively uncommon infectious outcomes of diseases for which the sample size required is driven up to achieve clinically useful statistical power.

Risks to be considered

Vaccine efficacy is calculated on a population basis. It is therefore relatively easy to misunderstand its application.

Cases studied

The NEJM did a study on the A flu efficacy Influenza virus. A total of 1952 subjects were enrolled and received study vaccines in the fall of 2007. Influenza activity occurred from January through April 2008, with the circulation of influenza types:

  • A (H3N2) (about 90%)
  • B (about 9%).

Absolute efficacy against both types of influenza, as measured by isolating the virus in culture, identifying it on real-time polymerase-chain-reaction assay, or both, was 68 percent (95 percent confidence interval [CI], 46 to 81) for the inactivated vaccine and 36 percent (95 percent CI, 0 to 59) for the live attenuated vaccine. In terms of relative efficacy, there was a 50 percent (95 percent CI, 20 to 69) reduction in laboratory-confirmed influenza among subjects who received inactivated vaccine as compared with those given live attenuated vaccine. subjects were placed in a healthy adult population. The efficacy against the influenza A virus was 72 percent and for the inactivated was 29 percent with a relative efficacy of 60 percent. The influenza vaccine is not 100% efficacious in preventing disease, but it is as close to 100% safe, and much safer than the disease.[4]

Since 2004, clinical trials testing the efficacy of the influenza vaccine have been drifting in: 2058 people were vaccinated in October and November 2005. Influenza activity was prolonged but of low intensity; type A (H3N2) was the virus that was generally going around the population, which was very alike to the vaccine itself . The efficacy of the inactivated vaccine was 16% (95% confidence interval [CI], -171% to 70%) for the virus identification end point (virus isolation in cell culture or identification through polymerase chain reaction) and 54% (95% CI, 4%-77%) for the primary end point (virus isolation or increase in serum antibody titer). The absolute efficacies of the live attenuated vaccine for these end points were 8% (95% CI, -194% to 67%) and 43% (95% CI, -15% to 71%).[4]


With serologic end points included, efficacy was demonstrated for the inactivated vaccine in a year with low influenza attack rates. Influenza vaccines are effective in reducing cases of influenza, especially when the content predicts accurately circulating types and circulation is high. However, they are less effective in reducing cases of influenza-like illness and have a modest impact on working days lost. There is insufficient evidence to assess their impact on complications.


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