Severe reactions to the Covid-19 vaccine are not common

By Stuff reporter

The Whole Truth: Covid-19 Vaccination | By Stuff reporter Hannah Martin


As the number of Pfizer/BioNTech Covid-19 vaccine doses given around the world increases at pace every day, reports of temporary, mild side effects after the jab are becoming commonplace.

The Pfizer vaccine has been provisionally approved for use in New Zealand in those over 16 by Medsafe, the organisation that regulates medicines in New Zealand. Medsafe has deemed the jab is both safe and effective.

However, the fact a vaccine is safe does not rule out some side effects. All medicines and vaccines carry the risk of some reaction.

People cite experiencing headache, dizziness, and nausea – common side effects with many vaccines, and in line with what was reported during clinical trials.

But with millions of people now vaccinated worldwide, reports of rare allergic reactions and other serious adverse events are emerging, including in New Zealand.

Early safety monitoring from the United States reported 21 cases of anaphylaxis from 1.8 million first doses of the Pfizer vaccine given in December 2020.

Anaphylaxis is a severe, life-threatening allergic reaction – occurring most often in those with a history of allergy.

In February, an explainer published in Nature stated the Pfizer vaccine triggers approximately five anaphylactic reactions per million doses, according to Vaccine Adverse Event Reporting System (VAERS) data.

This is a higher rate than most other vaccines – including annual flu shots, which trigger anaphylaxis for only one out of every million doses administered.

However, experts expect the rates for the Pfizer and other Covid-19 vaccines might change as more doses are administered.

It’s a numbers game. The more people vaccinated, the higher the chance we will see possible severe adverse reactions.

This is because trials involving tens of thousands of people cannot pick up rare side effects you might only see in one person per million doses.

This was the case with the risk of blood clots and the AstraZeneca vaccine, which New Zealand has not yet approved for use.

Data around the AstraZeneca vaccine suggests four to five people per million will experience rare, dangerous blood clots known as cerebral venous thromboses (CVTs). . There is no good data about the rate of these blood clots in the general population to know whether the risk among people who have had the Covid-19 vaccine is higher than the everyday risk of suffering a CVT.

It’s important to know that these blood clots have also been diagnosed in people with Covid-19 infections. A pre-print study (still to be peer-reviewed) found the risk of CVTs among people with Covid-19 was nearly seven times higher than the risk from being vaccinated.

To April 17, New Zealand’s Centre for Adverse Reactions Monitoring received 57 reports of serious adverse events after the Pfizer jab, from nearly 170,000 doses given — about three reports for every 10,000 doses.

Of these, 35 were allergic reactions and were treated appropriately. Two have been classed as possible anaphylactic events but neither was a level 1 reaction (the most serious end of the scale).

The centre reviewed two reports of blood clots and decided neither was related to the vaccination.

While there is a chance of a serious reaction to any vaccine, this is rare: you may walk away with a headache or sore arm — or most likely, with no side effects at all.

* Reporting disclosure statement: Professor David Murdoch and Dr. Maia Brewerton provided expert advice in the preparation of this article.

* This piece initially stated that the rate of serious blood clots among people who received the Moderna, Pfizer or AstraZeneca vaccines was four to five events per million doses given. The reference to Pfizer and Moderna has been removed as the data for blood clots connected to these vaccines is less certain than the data for AstraZeneca. The risk for all three remains low.