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How Rare is Rare?

We take a look at how the frequency of side effects is reported and what those terms actually mean

How Rare is Rare?

There is a lot of discussion at the moment about the potential side effects of vaccines.  We take a look at what terms such as “rare”, “common”, or “very common” actually mean in the context of side effects.

  • Very Common – A side effect that is reported as “very common” has a 1 in 10 or greater chance of happening.  For example,  if we gave a particular drug to 10 people we might expect to see at least 1 person experience a side effect that is classified as “very common.”
  • Common – “Common” side effects are those that have a 1 in a 100 chance of happening.  This means that we would if we gave the drug to 100 people we might expect 1 person to experience the side effect.
  • Uncommon  – “Uncommon” side effects are those that have a 1 in a 1000 chance of happening.  Once again, we would only expect to see an uncommon side effect once in every 1000 people given the drug.
  • Rare – “Rare” side effects have a 1 in 10,000 chance of happening.
  • Very Rare – The chance of  a very rare side effect is less than 1 in 10,000.  This means that, at the very least, we would only expect to see a side effect occuring once when more than 10,000 people are given the drug.  It may be that a side effect is 1 in a million or 1 in 10 million – it will still be classed as very rare.
  • Not Known – This means that the number of times a particular side effect might occur cannot be estimated from the data available.

Where does the information come from?

Information on the frequency of side effects is gathered during clinical trials.  Once a medicine goes onto the market, the drug company and health regulatory authorities continue to monitor for side effects.  Sometimes, very rare side effects are not noticed until a medicine is used in the general population.  Side effects or suspected side effects of medicines can be reported to the regulatory authority by anyone, including the patient.  Suspected side effects can be reported online to the Health Products Regulatory Authority (HPRA) here.

What else should I know?

Knowing the frequency of side effects does not give any information about their severity.  A very common side effect of a medicine may not really cause much harm for most people.  On the other hand a very rare side effect might have very serious consequences.

What’s the story with COVID vaccine side effects?

Understandably, we are all a little concerned about side effects from the COVID vaccines at the moment.  These medicines are new and, in some cases, involve new vaccine technologies.  Serious side effects can cause delays to the vaccine roll-out as health regulators examine what is going on.  Thankfully, most side effects are mild to moderate in nature and vaccines will continued to be used when the overall benefits outweigh the risks.