Why a sore arm after Covid-19 vaccination can be a good thing

By Stuff reporter

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


Like all medicines, the Pfizer/BioNTech Covid-19 vaccine, which is being used in New Zealand, may cause side effects in some people.

Most are mild and don’t last long. They’re most likely to kick in within a day or two after being vaccinated. 

When you get the Pfizer vaccine, little spheres of fat called lipids, containing mRNA (messenger ribonucleic acid), get taken up by cells in your arm muscle.

The mRNA gives your cells the recipe to start making a spike protein (the spiky bits you often see on drawings of Covid-19), so your body knows what the virus looks like. Your immune system sees the spikes and springs into action. One of the first things people experience when this happens is arm swelling and pain.

This is a positive thing: your immune system is building antibodies to protect against any future, real infection.

We know a lot about some side effects from clinical trials, which included more than 40,000 people.

As well as swelling and pain, the most common things people noticed were a headache and feeling tired.

Others experienced achy muscles, feeling generally unwell, chills, fever, joint pain, or feeling nauseous. 

These common side effects were reported by up to one in 10 people.

Uncommon side effects - which between one in 100 and one in 1000 people noticed - include enlarged lymph nodes, feeling unwell, limb pain, not being able to sleep, and itching at the injection site.

Some people don’t feel side effects but that doesn’t mean that the vaccine hasn’t worked for you: the Pfizer jab is still 95 per cent effective

While you might feel sick, the vaccine cannot give you Covid-19, as mRNA vaccines do not contain any weakened or dead parts of the virus. 

Serious allergic reactions do happen, but are extremely rare and can be treated. They usually happen soon after you’ve had your vaccine, which is why you need to wait at the doctor or vaccination site for at least 20 minutes after you get it. 

Medicines and vaccines are monitored closely for their safety. 

In Covid-19 vaccine trials, people were monitored for side effects for two months after receiving their second dose but people who were part of the Pfizer trials will continue to be monitored for the next two years. 

In New Zealand, adverse events following vaccination are reported to the Centre for Adverse Events Monitoring (CARM). A report does not necessarily indicate a link between the symptoms and the vaccine, just that the symptoms were noticed in the hours or days after vaccination.

As of July 3, data from Medsafe shows that out of 1,229,212 doses given, there have been 6145 reports. Of those, 5877 were non-serious, with headaches, dizziness, and nausea the most common reports. 

Of the 268 serious events, 21 were considered possible anaphylaxis (severe allergic reaction). Medsafe has ruled out a connection between the vaccine and 14 deaths reported to CARM. Two more deaths are still being investigated and CARM did not receive enough information to assess a further two deaths. 

Other serious events included reports of flu-like symptoms, possible stroke, hearing loss or tinnitus, and myocarditis.

Reporting disclosure statement: This post was reviewed by The Whole Truth: Covid-19 Vaccination expert panel members Dr Maia Brewerton, a clinical immunologist, allergist, and immunopathologist; and Professor David Murdoch, clinical microbiologist and infectious diseases expert, and Dean of the University of Otago, Christchurch.