Vaccine efficacy

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Vaccine efficacy or vaccine effectiveness is the percentage reduction of disease cases in a

The

The basic formula^[7] is written as:

$${\displaystyle VE={\frac {ARU-ARV}{ARU}}\times 100\%,}$$

• ${\textstyle VE}$ = Vaccine efficacy,
• ${\displaystyle ARU}$ = Attack rate of unvaccinated people,
• ${\displaystyle ARV}$ = Attack rate of vaccinated people.

An alternative, equivalent formulation of vaccine efficacy is:

$${\displaystyle VE=(1-RR)\times 100\%,}$$

${\displaystyle RR}$

The design of

In these cases, the formula would yield a negative efficacy value because ${\displaystyle ARV>ARU}$. A negative efficacy value is sometimes present in the lower limit of a

Relative risk

First, the baseline risk can be calculated for each group and then vaccine efficacy (RRR) as follows:

• ${24 \over 12221}=0.196\%$ for the vaccinated group (24 infections)
• ${\displaystyle {106 \over 12198}=0.86\%}$ for the placebo group (106 infections)
• The relative risk, ${\displaystyle RR={0.196 \over 0.86}\approx 0.23}$

Then, ${\displaystyle VE=(1-RR)\times 100\implies (1-0.23)\times 100\approx 77\%}$

Also, the

Testing

Vaccine efficacy differs from vaccine effectiveness in the same way that an explanatory clinical trial differs from an intention-to-treat trial^{[clarification needed]}: vaccine efficacy shows how effective a vaccine could be given ideal circumstances and 100% vaccine uptake (such as the conditions within a controlled clinical trial); vaccine effectiveness measures how well a vaccine performs when it is used in routine circumstances in the community.^[9] What makes vaccine efficacy relevant is that it shows the disease attack rates as well as a tracking of vaccination status.^[jargon][9] Vaccine effectiveness is relatively inexpensive to measure than vaccine efficacy. The measurement of vaccine effectiveness relies on observational studies which are usually easier to perform, whereas a vaccine efficacy measurement requires randomized controlled trials which are time and capital intensive.^[10][9] Because a clinical trial is based on people who are taking the vaccine and those who are not, there is a risk for disease, and optimal treatment is needed for those who become infected.

The advantages of measuring vaccine efficacy is having the ability to control for

The New England Journal of Medicine did a study on the efficacy of a vaccine for the influenza A virus. A total of 1,952 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

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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