Israeli research explains post-vaccine grogginess, and why it’s not worrying

First digital monitoring of COVID shot recipients shows vitals spike but return to normal within three days, demonstrating ‘vaccine safety’

Nathan Jeffay is The Times of Israel's health and science correspondent

A medical worker prepares a COVID-19 vaccine injection, at a Clalit vaccination center in Jerusalem, on March 8, 2021. (Yonatan Sindel/Flash90)
A medical worker prepares a COVID-19 vaccine injection, at a Clalit vaccination center in Jerusalem, on March 8, 2021. (Yonatan Sindel/Flash90)

COVID-19 vaccines cause a “dramatic” short-term change in recipients’ bodies, but they quickly go back to normal, Israeli researchers have found, in a first-of-its kind digital monitoring project.

Their study gives a physiological explanation for why many people feel groggy after their second vaccine, showing that it sends heart rates racing, raises blood pressure and causes various other changes to vital statistics.

But rather than worrying people, the close-up view of reactions should reassure them, said Prof. Dan Yamin of Tel Aviv University.

That is because many around the world are hesitating over vaccines, concerned that others have experienced tiredness and weakness after shots, and sometimes regarding those symptoms as inexplicable and worrying that they could point to an unreported health risk associated with vaccines.

An Israeli man receives a COVID-19 shot at a vaccination center in Jerusalem, on March 11, 2021. (Yonatan Sindel/Flash90)

Yamin said that his study has removed the mystery from what causes people to feel somewhat unwell after vaccines, showing that it results from straightforward physiological reactions that pass quickly.

“We showed that there are dramatic and significant physiological changes after people receive the second vaccine shot,” said Yamin. “But at the same time, we show there are no changes so large that they are cause for concern, and none of the changes caused harm to the patients.

Prof Dan Yamin of Tel Aviv University (Moshe Baderashi)

“We also show that changes in vital stats are more apparent in younger than older people and — importantly — that they only last for three days, after which they return to pre-vaccination levels.”

He noted that changes were experienced by almost all of the vaccinated Israelis he followed. Those who felt tired or unwell following the vaccine had the highest levels of the various vitals that were measured, but raised levels were also seen among those who felt normal, he said.

Oxygen saturation levels, which should remain stable to maintain good health, did not fluctuate after vaccination, Yamin stressed.

The study, which has been posted online but not peer reviewed, did not set out to asses the occurrence of reported side effects. This was achieved in a recent peer-reviewed study in The Lancet, which found that side effects reported by recipients of the Pfizer vaccine in the real world are rarer than in clinical trials. Rather, the new study set out to chart the effect of vaccines on the body using digital monitoring, rather than self-reporting.

It followed the stats of 160 Israelis aged 18-plus after they received their second Pfizer coronavirus shot. Yamin’s team fitted them with special pads that transmitted data through a smartphone app.

Stats peaked at 20 hours after vaccination, at which point heart rate was an average of nearly 10 percent higher than pre-vaccination levels. Diastolic blood pressure was almost 5% higher than pre-vaccination rates, as was the respiratory rate. Levels then dropped, and returned to pre-vaccination rates by three days after the shot.

“This is another demonstration of the safety of the vaccine, from a standpoint that has not been explored before, namely digital monitoring,” said Yamin. “We see that our body produces an immune response to the vaccine, as it should, and the fact that the vital statistics in our body then change slightly is just a result of this happening.”

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