Vaccine leaflets: understanding ‘frequency not known’

In the cited flu vaccine product information, ‘frequency not known’ means that the frequency cannot be estimated from the available data. This wording does not automatically classify the effect as rare, nor does it mean the effect is common.

An estimated category has boundaries. For example, the same document defines ‘rare’ as at least one case in 10,000 and fewer than one in 1,000. This classification convention differs from having no estimate; it is not a personal prediction for your next injection.

Also check the population concerned. The leaflet specifies that certain effects have an unknown frequency in the general population, except in a group where that effect is already listed with a frequency. A value for one group must not be extended to every reader of the leaflet.

Spontaneous reports help identify suspected effects and safety signals. In its methodological report, ANSM explains that they cannot, on their own, estimate occurrence frequency: reported cases do not provide exhaustive follow-up of everyone vaccinated.

Dividing the number of reports by a number of people or doses does not automatically solve this limitation. ANSM specifies that even a reporting rate calculated with an appropriate denominator must not be read as an incidence rate, notably because of underreporting.

To understand a statement, keep the exact leaflet, the population concerned and the origin of the figure: a clinical study or spontaneous reporting. Ask the pharmacist or doctor what can actually be estimated. An unknown frequency does not let you invent a percentage or decide alone what to do about a symptom.