Why the second Covid-19 vaccine packs a bigger punch

By Stuff reporter

The Whole Truth: Covid-19 Vaccination | By Stuff reporter Brittney Deguara.

We already know people can experience mild side effects in the days following the first Covid-19 jab - in fact, a sore arm can be a good sign. 

But some people report feeling worse after the second jab. Why? It’s not what’s being injected because both doses of the Covid-19 vaccine are exactly the same, rather it’s how your body responds to these injections.

The first Covid-19 vaccine dose teaches your cells how to respond to the SARS-CoV-2 virus, the one that causes Covid-19, should they ever come into contact with it. Think of it as a 101 class teaching your body to fight back against the disease. 

When it comes time for the second Covid-19 dose, your body already knows what to look for, so it comes back with a bigger and better immune response. This is why some people experience greater - though almost always still mild - side effects the second time around. 

This doesn’t only happen with Pfizer’s Covid-19 vaccine, it’s also true for the tetanus shot.

The Covid-19 vaccine dosage, prior exposure to the vaccine or the disease and whether an adjuvant (an ingredient used to create a stronger immune response, which Pfizer’s Covid-19 vaccine has) is included can all impact upon the side effects. 

In the clinical trials of the Pfizer Covid-19 vaccine, and in real-world studies that followed, headaches, fatigue, and injection site pain were commonly reported. 

Researchers in the US analysed data from the CDC’s V-Safe portal of millions of people who received either one or both doses of the Pfizer and Moderna Covid-19 vaccines - these are both mRNA vaccine types. 

Injection site pain and swelling, fatigue, headaches, chills, fever, and joint pain were more commonly reported after the second vaccine dose than the first.

The research concluded mild side effects were “substantially greater” after the second jab and were most common one day after vaccination. 

Those over 65 years of age weren’t as affected by the second Covid-19 jab compared to younger participants - this is quite common for other vaccines because immune systems weaken with age and don’t produce as many T cells (the cells that vaccines teach how to fight off infections). Despite this, all age groups still reported increased reactions. 

Studies have also looked at the side effects after Pfizer Covid-19 booster shots with most people experiencing similar side effects to the second dose.

It’s important to clarify that side effects aren’t necessarily a given, and that serious adverse events are very rare. 

In general, side effects don’t always occur and if they do they tend to be mild or moderate and resolve within 24 to 48 hours. 

Severe allergic reactions and myocarditis don’t appear to be more common after the second Covid-19 jab. 

Dr Frances Priddy, executive director of Vaccine Alliance Aotearoa New Zealand explained both of these events are “very uncommon” and are almost always “treatable” if they do occur.

Reporting disclosure statement: Dr Frances Priddy, the executive director of Vaccine Alliance Aotearoa New Zealand, provided expert advice for this post. It was also reviewed by The Whole Truth: Covid-19 Vaccination expert panel member Dr Dianne Sika-Paotonu, an immunologist and senior lecturer in pathology and molecular medicine at the University of Otago.