A new study of about 48,000 people in the United States found that third-dose mRNA vaccines, such as those from Pfizer or Moderna, were safe and did not lead to any increase in the reporting of serious adverse events.
A study published in the JAMA Network Open on Thursday found that third doses did not increase severe side effects compared to earlier doses, but came with an increase in low-severity side effects such as fatigue and nausea.
“The results of this cohort study suggest that a third dose of the same type of vaccination after a primary series of BNT162b2 or mRNA-1273 has been associated with safe results,” the study said.
The COVID-19 vaccines, manufactured by Pfizer-BioNTech and Moderna, were originally designed as two-dose regimens, supported by extensive clinical trials. As studies have shown evidence of reduced immunity to the vaccine, health experts have begun offering booster vaccines that have been found to be safe and effective. But there is still less data on these third doses than on the first and second doses, a knowledge gap that the researchers hoped to close in this new study.
Researchers gained access to electronic medical records provided by 47,999 adults at Mayo Clinic Enterprise, a non-profit hospital system in the United States, all of whom were vaccinated with three doses of the same mRNA vaccine against COVID-19 between December 2020. and October 17, 2021.
About 38,000 participants received the Pfizer-BioNtech vaccine for their three doses, while about 9,900 received the Moderna vaccine.
There were more than twice as many immunosuppressed individuals in the Moderna group as in the Pfizer group. No brand-specific adverse events were reported in the study.
The most common side effects after the third dose are fatigue, temporary swollen lymph nodes, nausea and headache, and these symptoms are reported in about two to five percent of participants.
Less than two percent of participants reported other common side effects such as joint pain, muscle aches, diarrhea and fever.
These figures, although still small, represent a significant increase in how often these same participants reported these side effects after the first or second dose.
After the third dose, two patients had anaphylaxis, four patients had swelling of the tissue around the heart called pericarditis, and one patient reported myocarditis, meaning that only 0.01% of participants reported one of these serious side effects.
“Reporting of severe adverse reactions is rare after the third dose and does not increase significantly compared to the frequency of reporting after the second dose,” the study said.
The researchers also monitored emergency room visits for participants within two days of receiving each dose of the vaccine and found that there was an increase in emergency room visits after the third dose of Pfizer vaccine compared to the second. Overall, 0.29% of people visited the emergency department after the third dose versus 0.2% after the second dose.
There was no difference in ER visits for different doses for those receiving Moderna.
Overall results show that third doses are indeed safe, as they do not lead to an increase in severe side effects, the researchers said. They added that the third dose had a better safety profile in this study than receiving a single dose.
“Persons who received exactly [one] dose of COVID-19 mRNA-based vaccine has a greater increase in the incidence of adverse events after dosing [one]compared to baseline than that observed at [three]”the dose cohort,” the study said.
Researchers note that there are limitations to the study, such as the fact that people receiving mixed brands are not included and that those receiving third doses are more likely to be older and immunosuppressed, thus potentially having reduced immune response to the vaccine, which may show fewer side effects.
We hope that further third-dose studies will be able to shed more light on the safety profile of mixing vaccine brands to further narrow the frequency of rare adverse events, the study said.
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