VMD changes to calculation methods ensures figures reflect the way medicines are used
The Veterinary Medicines Directorate (VMD) explains changes to how it calculates adverse event incidence rates for animal medicines, following updates to Veterinary Medicines Regulations in 2024. The changes will help to ensure the figures we report give a clear picture of real-world animal medicine use, making them easier to understand and compare.
The changes mean that previously published figures may be updated using new calculation methods, which could make them look very different to earlier versions - even if the actual number of adverse events reported hasn't changed.
For example, under the previous method an incidence rate might have been shown as 0.0001, while under the new method we might report an incidence rate percentage of 0.01%. Both figures represent the same rate, but the percentage appears to be 100 times larger.
What are adverse events?
No medicine is 100% risk-free. Adverse events are observations that are unfavourable and unintended that happen after giving a medicine to an animal.
These events may be seen, for example, in the animal being treated, in other animals, the person handling the medicine or in the environment.
They include both reactions and times when the medicine has not worked as expected and can range from mild issues to more serious problems.
New reporting requirements for pharmaceutical companies
Under the updated regulations, companies that hold marketing authorisations for animal medicines must provide adverse event incidence percentages when submitting safety information to the VMD using the new calculations. Details of the calculations to be used can be found in section 4.4 of Guideline IV Signal management, including benefit-risk reports - GOV.UK. Companies must also supply sales data at least yearly, which the VMD will use to estimate how many animals have been treated with each product.
How the VMD will now calculate adverse event incidence rates
The VMD will now calculate adverse event incidence rates using the formula:
Number of animals experiencing adverse events divided by the estimated number of animals treated, multiplied by 100 to give a percentage.
To estimate how many animals have been treated, the VMD will use sales figures combined with "dose factors" that account for how products are typically used according to their official instructions. This method is used as there is no system in place to gather usage data and recognises that some medicines require multiple doses per animal. This new method ensures adverse event incidence rates reported by the VMD reflect the way these medicines are used in practice.
The VMD has also changed which dates it uses for calculations. Where possible, it will now use the date when an adverse event started, rather than when it was reported to the VMD. This change aims to better reflect that some adverse events are reported long after they occur, however this approach is not suitable in all situations . Where this approach is not suitable, the VMD will give details on how a figure was calculated when publishing.
