Vaccine efficacy

  • by VaccineHealthCenter

Vaccine efficacy refers to the percentage by which a disease reduces among the people who receive vaccination in a clinical trial. Therefore, vaccine efficacy is an indication of how well the vaccine can work, given that the conditions are most favorable and the uptake is up to 100%. Vaccine efficacy is carried out under controlled conditions, offering the ‘best’ or the ‘most favorable’ conditions unlike vaccine effectiveness which measures how the vaccine works among people who receive the vaccination outside a clinical trial, vaccine efficacy closely monitors how the vaccination impacts disease reduction for the people who receive the vaccination in the clinical trial.

  When is vaccine efficacy done?

When a new vaccine is about to be released into the market, the health agencies and regulatory bodies subject the vaccine candidate to clinical trials. For every vaccine clinical trial, there are specific goals that the trial seeks to fulfill. A clinical trial study protocol is conducted to determine the goals of the clinical trials. Such goals are conventionally known as endpoints. In determining the vaccine efficacy of a given vaccine candidate, there have to be vaccine efficacy endpoints. For the Covid-19 pandemic, the efficacy endpoints of most vaccine candidates revolve around preventing new cases of symptomatic Covid-19 infections.

Why is vaccine efficacy significant?

There is no single vaccine that can be dispatched to the population without going through clinical trials to determine their vaccine efficacy. This is because a vaccine efficacy provides relevant information about the vaccine before it can be released to the market and the target population. The rates of spread, cost of the vaccine, hospitalization cases, and medical visits can be determined before a vaccine is released to the public using vaccine efficacy.

How is vaccine efficacy determined?

In most cases, health agencies work out a vaccine efficacy by monitoring two groups, one that receives a placebo, and the other that receives the vaccine under trial. The agency collects the data for new cases of the disease from both groups. By considering the difference in the new cases in the two groups, the vaccine efficacy can be determined. Placebo refers to a medicine or a prescription that a patient receives for his psychological benefits as opposed to physiological benefits. When a patient takes a placebo in the form of inert injections or pills, there are no therapeutic results to be expected. The placebo, placebo-controlled trials, and the data obtained from those taking the vaccine in the trial stage are used to determine the vaccine efficacy.

Calculating vaccine efficacy

In determining how well a vaccine can work under ideal conditions with a 100% intake, a specific formula is used. Below is the vaccine efficacy formula;
Text Box: VE= {(ARU-ARV)/ARU} *100%  Whereby, ARU= attack rate among the unvaccinated group ARV= attack rate among the unvaccinated group   VE= Vaccine Efficacy

What happens during the determination of vaccine efficacy?
In determining a vaccine efficacy, health agents closely monitor the volunteers taking the trials in the clinical stages. The health researchers keep a track of the participants’ general health, and any medical condition that develops is recorded. In case the participants notice any side effects, they report to the researchers. In some cases, the trial volunteers are given diaries that they fill and record the monitoring symptoms.

Vaccine efficacy exclusions

The vaccine efficacy is an index that is only determined during the stages of the clinical trial of a vaccine candidate. In these trials, some exclusions are made. The exclusion criteria are the properties of a clinical trial that exclude certain members of a population from the clinical trials. Such exclusions include age, pregnancy, and some specific medical conditions. Pregnant women, seniors, small children, and people with weak physiological health are normally excluded from most clinical trials that determine vaccine efficacy.

The risks associated with vaccine efficacy

Although vaccine efficacy is such a critical index and an elementary prerequisite for a vaccine candidate’s approval, there are risks associated with it. First, the efficacy is established using data obtained from controlled conditions. Realistically, the data obtained from controlled conditions usually vary from the data obtained from the natural environment. This may mean that the vaccine efficacy will not reflect the true values in typical environments. Secondly, the exclusion criteria sometimes cut out certain members of the population, not including them at all. Unknown to the researchers, the results may turn out differently on the excluded population from what was pre-empted. Again, this subjects the vaccine efficacy datum to bias.


Before the regulatory bodies can accept a vaccine candidate into the market for public dispatch, the responsible company has to determine the vaccine efficacy. This is a measure of the percentage disease reduction of the vaccinated group compared to the unvaccinated group. The vaccine efficacy helps determine a lot of information about a vaccine before release, making it an essential parameter. However, vaccine efficacy can have biases from exclusion criteria and the variation in controlled conditions and natural conditions.