WASHINGTON (BLOOMBERG) - People who got Covid-19 vaccine boosters after the shots were cleared for those with weakened immune systems had mostly mild to moderate reactions, according to the US Centres for Disease Control and Prevention.

Some 22,191 booster recipients registered with a federal monitoring system starting Aug 12, when the doses were first authorised on an emergency basis by US regulators, through Sept 19, the CDC said in a report on Tuesday (Sept 28).

Overall, 71 per cent reported pain at the injection site, while 56 per cent reported fatigue and 43 per cent said they experienced a headache after getting a third dose of a messenger RNA vaccine.

The data were reported by recipients zero to seven days after a third dose. Regulators opted to make third doses of the shots made by Pfizer-BioNTech and Moderna available last month after data suggested that transplant recipients and other people with immune-system deficiencies were not being given adequate protection by the conventional two-shot regimen.

Last week, the Food and Drug Administration also authorised a third dose of the Pfizer vaccine for a broader group. Additional shots for recipients of the Moderna and Johnson & Johnson vaccines could be cleared soon, health officials have indicated.

Among those who reported their reactions to the monitoring system, about 1.8 per cent said that they sought medical care, while 13 people said they were hospitalised.

Specific reasons for receiving care were not reported in the survey.

"The frequency and type of side effects were similar to those seen after the second vaccine doses, and were mostly mild or moderate, and short-lived," CDC director Rochelle Walensky said at a briefing by the White House Coronavirus Task Force on Tuesday.

Little mixing

The patterns of adverse reactions after a third dose for both the Moderna and Pfizer-BioNTech shots were consistent with those that have been described after a second dose. The so-called primary series for both of the messenger RNA vaccines involves two doses administered several weeks apart.

Debate over the merits of taking vaccine doses from more than one drugmaker has been growing since last week's Pfizer clearance, but there was not enough data to determine if mixing causes adverse reactions.

Nearly all of those who reported to the monitoring system said they stuck with the same manufacturer for all three doses.

The findings are limited to people who self-reported. A majority identified themselves as White and non-Hispanic.

Overall, approximately 2.21 million people in the US had received additional vaccine doses as of Sept 19, according to the CDC.