The case of two aged care residents mistakenly being given four times the recommended dosage of the Pfizer COVID-19 vaccine in the first week of Australia’s rollout is not without precedent, but nor is it a cause for panic.
Federal Health Minister Greg Hunt said the pair — an 88-year-old man and a 94-year-old woman — were being monitored and were “showing no signs at all of an adverse reaction”.
In Germany’s Pfizer vaccine rollout, eight aged care workers received up to five times of the recommended dose of the Pfizer jab. Four were later hospitalised after developing flu-like symptoms.
All eight workers went on to recover without any adverse effects.
But what happens to our body if we do get a larger dose?
Here’s what the science tells us so far.
Pfizer’s early vaccine trials
The Pfizer vaccine contains genetic material called mRNA (which encodes the SARS-CoV-2 spike protein), plus a handful of pharmaceutical ingredients that help to stabilise it.
Australia’s Therapeutic Goods Administration granted provisional approval in January for use of the Pfizer vaccine, outlining that it requires two 30-microgram doses, to be taken 21 days apart.
In the vaccine’s initial trials, doses of up to 100mcg were given to participants without serious side effects. (Other participants in those early trials were given a 10mcg dose, 30mcg dose or placebo.)
Researchers then weighed up the efficacy rate of the drug against the likelihood of an adverse event, before setting the vaccine’s optimal dosage.
As University of Queensland professor of medicine Paul Griffin explains, there is a “very high margin of safety” with the dosage amounts used in clinical trials.
“We nearly always test doses that are higher than we ultimately use, again so we can be confident of that safety margin in the vaccines that we are using.”
But it’s worth noting that people in Pfizer’s early trials who had the higher dose were more likely to report redness or swelling at the injection site and severe pain than participants receiving lower dosages.
Professor Griffin said participants who were given the higher dosage also had a higher chance of experiencing systemic effects such as flu-like symptoms (similar to the German health workers’ reaction).
Will the two residents be OK?
If these residents received four times the recommended 30mcg dosage, they would have been given 120mcg of the vaccine, more than Pfizer tested in its initial trials.
Murdoch University professor of immunology Cassandra Berry explained that the residents would have received more mRNA — the genetic blueprint for the SARS-CoV-2 spike protein.
But this might not necessarily be a bad thing, Professor Berry explained, and the larger dosage might even “work better” than the optimal dose of 30mcg.
“There will be more protein made, which is the spike of the coronavirus, and the body is quite used to preparing lots of [mRNA] into protein every day,” she said.
And Professor Griffin said judging by the amount the residents were given, the prospect of ill effect was “fortunately very low”.
Despite the residents’ age, they’re no more likely to develop negative side effects from the higher dose than younger counterparts.
While it was “hard to say” how much of the Pfizer vaccine a person would have to be given to result in serious side effects, Professor Griffin said it would be “significantly higher” than 120mcg.
Giving older people larger vaccine doses is standard too, Professor Griffin added.
“We know with some vaccines, people who are elderly actually respond [and develop immunity] a bit less than people who aren’t.
“With the flu, we have a vaccine for people that are elderly with a higher dose for that season.”
If a larger dose works better, why won’t we get it?
Well, because results still show a dose of 30mcg with a second 30mcg booster is the optimal dosage — and is the amount the TGA recommends.
There’s also the fact that we’re also in a global pandemic.
“What happens when there is a crisis and there is a limited stockpile of vaccine, they [regulatory bodies and governments] go into dose-sparing strategy,” Professor Berry said.
“If we give people a higher dose … we run out of a manufacturing supply base.”
In the first round of Pfizer drug trials, participants who received the 100mcg dose weren’t given a second 100mcg dose.
But Professor Berry said even if you had a higher first dose, you would still need a second shot.
“You definitely need the booster shot to prime the cells to make the antibody [to protect against the virus],” she said.
“The antibody is what floats around in your blood and that would bind to the spike protein of the virus, if you were to see it in real life, and it neutralises the virus.”
In Australia, anyone who is given an incorrect dose will be monitored closely by health authorities, as will anyone who reports adverse or unusual effects to the drug itself.
What next?
Professor Griffin said while it was “really disappointing” this dosage error had occurred in the first week of the rollout, people could still have faith in the vaccine program.
He was concerned that reports of the incident could fuel some people’s anxiety about the Pfizer rollout.
“It’s important to know it was transparently reported in real time, and that the vaccine was proven safe and effective in clinical trials,” he said.
Professor Berry agreed, noting that for a vaccine to be provisionally approved by the TGA, it must pass considerable testing — on top of the thorough three-phase testing that Pfizer put the vaccine though.