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Are adverse events in Covid-19 vaccine trials under-reported?

By Maryanne Demasi, PhD

Phase III controlled trials are considered the ‘gold standard’ for assessing vaccine efficacy and safety in humans.

These are the trials upon which authorities have assured the public that Covid-19 vaccines are “safe and effective.”

The benefits of the vaccines in reducing disease severity and hospitalisation are well-known, but what do we know about the adverse events?

Data on adverse events is vital for effective decision-making by regulators, policy makers, doctors, and patients.

But there are serious concerns about publication bias or selective omission of data, whereby adverse events are less likely to be published than positive results.

A systematic review in PLOS journal analysed at 28 studies and found that adverse events were less likely to be published in studies appearing in journal articles compared to unpublished studies (e.g. industry-held data).

Experts now suggest that the pivotal Covid-19 vaccine trials may have under-reported vaccine-related adverse events in several ways.

Virtual monitoring with digital apps

In the Pfizer and AstraZeneca vaccine trials, participants were given digital apps to record adverse events remotely - a more convenient, time efficient and cost-effective way of gathering patient data.

A major problem however, is that the pre-determined options on the digital apps have a narrow focus on particular adverse events.

For example, the app only allows a participant to record what the company deems as ‘expected’ events such as fever, pain at injection site, temperature, redness, swelling, fatigue, headache, diarrhoea, chills, muscle and joint pain.

But if they experience a serious adverse event like myocarditis or early signs of transverse myelitis, Guillain-Barre Syndrome, a myopathic disorder, myocarditis or thrombosis, there is no option for them to record it on the app.

Case in point: Brianne Dressen, a participant in the AstraZeneca (AZD1222) trial. She suffered a severe adverse event after her first injection and became disabled.


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