Young people and women more likely to report Pfizer, AstraZeneca COVID vaccine side effects, UK survey shows
Younger people, women and those who previously had COVID-19 are more likely to report side effects after getting vaccinated with the Pfizer or AstraZeneca vaccines, according to a survey in the UK.
Key points:
- One in four people who participated in a UK survey reported a systemic effect such as fever and headache and two in three reported a local effect such as a sore arm
- Most side effects occurred within 24 hours of getting the vaccine, with symptoms typically resolving within 48 hours
- Early Australian data shows half of people who have been vaccinated (and responded to the survey) reported an adverse event, mostly a sore arm
The survey, published in the medical journal The Lancet, relied on self-reports and data from 627,383 people who received either one or two doses of Pfizer or the first dose of AstraZeneca.
Most side effects occurred within 24 hours of getting the vaccine, with symptoms typically resolving within 48 hours.
La Trobe University immunologist Stephanie Gras said side effects post-vaccination were “really normal” if symptoms were mild and didn’t last long.
Overall, fewer adverse events were reported in this real-world observational study than in Pfizer and AstraZeneca’s phase 3 clinical trials.
Side effects post-vaccination
Researchers split the nature of side effects reported into two categories: local and systematic.
Local effects are those near the site of injection such as pain, swelling, redness, itching and swollen armpit glands.
Systemic effects are those that affect the entire body and include symptoms such as diarrhoea, fatigue, headache, chills, and nausea.
Photo by Victoria Jones/PA Images via Getty Images
)In total, one in four people who participated in the UK study (via an app) reported one or more systemic effects, and two in three people reported one or more local effects.
Nicholas Wood from University of Sydney’s faculty of medicine and health said people reported more adverse reactions to the second dose of Pfizer vaccine than the first.
“In the Pfizer, the first dose primes you, it’s the first time you’re seeing that particular antigen when the spike protein is made in your body,” he explained.
“When you see it the second time around, you already have some antibodies, and you are more likely to have a side effect.”
And while this study did not compare the first dose of AstraZeneca with the second dose (it only looked at reports after the first), Dr Wood said clinical trials showed adverse effects were more common after the first dose.
Age, BMI, comorbidities and vaccination
The study also split respondents up into several categories including body mass index (BMI), if they had any other health conditions (such as cancer and kidney disease), whether they’d previously had COVID-19 disease, and age.
Researchers found younger people, women and those who have previously had COVID-19 were more likely to report having had side effects.
Dr Wood says one reason younger people may report more symptoms is they have “robust” immune systems.
“We think these side effects are related to your immune system recognising the vaccine as a foreign antigen and stimulating the immune response,” he said.
Older people receive a higher dose of the flu vaccine, for example, because they need more of it to develop the same immunity as younger people.
But Professor Gras also hypothesised that older people could simply have a higher tolerance for pain or be less likely to report adverse events on an app.
BMI and reported comorbidities didn’t make a huge difference, although those with a higher BMI were slightly more likely to report a systemic reaction after the Pfizer vaccine.
Professor Gras said this was interesting, considering both BMI and comorbidities were factors in people developing more severe types of COVID-19.
Those who previously had COVID-19 were roughly three times as likely to report systemic effects (such as a headache or fever) after the first dose of the Pfizer vaccine and nearly twice as likely to report one after the second dose of Pfizer, or first of AstraZeneca.
Dr Wood said it wasn’t clear why.
“The thinking is that you already have some immunity from having had the wild virus,” he said.
Dr Wood explained people who had been previously infected already had antibodies against the virus, which meant they reacted to the vaccine as if it was fighting off a second infection.
How likely was infection post-vaccine?
The study also looked at infection rates, comparing those who responded to the survey with an unvaccinated control group.
It found both vaccines decrease the risk of SARS-CoV-2 infection after 12 days.
“The vaccines are extremely efficacious, they work very well and protect people from severe forms of COVID,” Professor Gras said.
Dr Wood agreed.
“The [Oxford-Astrazeneca] vaccine could reduce infection by 60 per cent [after a single dose], and for around about 70-odd per cent after one dose of the Pfizer vaccine,” he said.
The observed rate of infection in the study was on par with clinical trials.
UK study’s limitations
This study relied on self-reports, so even though it included a large sample size, there were some limitations.
“One of the issues with that is it’s not a controlled environment,” Professor Gras said.
“Usually if you do clinical trials, or if you do medical research, you try to control all parameters.”
Professor Gras also said clinical trial participants were typically more aware of the potential medical side effects than the general population.
Scientists are also able to weigh up reports of side effects in clinical trials to factor in differences in people’s sensitivities and pain thresholds.
On the flip side, Dr Wood said clinical trials focused on testing vaccines on healthy people, whereas this real-world study included people who had health conditions such as cancer and lung disease.
“The safety data that we see in trials may not completely reflect what’s happening when it rolls out in the real world, when we do start to vaccinate people who have underlying medical issues,” he said.
Dr Wood said while the UK study wasn’t totally representative of the whole population, it was a good indication.
“But they’ve got a large number of people enrolled, and they do have people over 55 and with underlying comorbidities, so it’s getting close to being representative of the whole community.”
Interestingly, Dr Wood said the UK data told a different story to results coming out of the US and early results from Australia.
The US is using a self-reporting system called v-safe and is currently vaccinating people with Moderna and Pfizer, not AstraZeneca.
“They’ve got over a million participants in that [v-safe] system [in the US] and they’re reporting … more common things like injection site pain muscle aches, and fever etc, than the UK experience,” he said.
Australia uses a system called AusVaxSafety, which to date has had 325,522 participants in its survey.
Of that figure, 52.4 per cent of Australian participants reported an adverse event, the most common of which was pain at the injection site.
Dr Wood hypothesised that people in the UK could be less likely to report adverse events than people in the US and Australia, but said it was really not clear why there was a difference at this stage.
Both experts agreed the UK study showed overall both Pfizer and AstraZeneca vaccines were safe and effective, and urged people to get vaccinated when eligible.