Special report on background incidence of adverse events in vaccine studies

Simon Collins, HIV i-Base

Hearts flanking a virusThe background incidence of the 15 most important and serious adverse events associated with vaccines against COVID-19 have been analysed from a multinational cohort study and published as a special report in the BMJ.

This includes records from more than 126  million people from 13 databases in the three years before COVID-19 who were observed for at least a year. Results stratified by age and sex.

The events included non-haemorrhagic and haemorrhagic stroke, acute myocardial infarction, deep vein thrombosis, pulmonary embolism, anaphylaxis, Bell’s palsy, myocarditis or pericarditis, narcolepsy, appendicitis, immune thrombocytopenia, disseminated intravascular coagulation, encephalomyelitis (including acute disseminated encephalomyelitis), Guillain-Barré syndrome, and transverse myelitis.

Older age generally increased the risk of most events although anaphylaxis, narcolepsy and appendicitis were exceptions more common at younger ages. The incidence of most events (reported as rates per 100,000) varied significantly between different databases.

For example, incidence of Bell’s Palsy ranged from 4 to 174/100,000 patient years in an Italian and US database, respectively. There was also a 3-fold difference between the highest and lowest incidence rates for deep vein thrombosis in each database.

The tables compiling these results are notably impressive and are and important reference resource.


Li X et al. Characterising the background incidence rates of adverse events of special interest for covid-19 vaccines in eight countries: multinational network cohort study. BMJ 2021;373:n1435. doi: 10.1136/bmj.n1435 (14 June 2021).

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