Covid booster shots lose much of their potency after about four months, raising the possibility that some Americans — specifically those at high risk of complications or death — may need a fourth dose, data published on Friday by the Centers for Disease Control and Prevention suggest.
Preliminary research from Israel and Britain has hinted that protection from booster doses declines within a few months. The data released on Friday offer the first real-world evidence of the mRNA shots’ waning power against moderate to severe illness in the United States.
The analysis did not include a breakdown by age, and the researchers could not distinguish between a booster shot or a third dose given to an immunocompromised person as part of the primary series.
The study focused on people who sought medical care for symptoms of Covid, so if that population was skewed toward older adults or those who have weak immune systems, the booster shots may have seemed less effective than they really are.
Other studies have shown that while vaccines may lose some ability to prevent severe illness and hospitalization in adults older than 65, they remain highly protective in younger adults in good health. Federal health officials will need to know who exactly is at high risk even after three doses before considering recommending a fourth shot.
“There may be the need for yet again another boost — in this case, a fourth-dose boost for an individual receiving the mRNA — that could be based on age, as well as underlying conditions,” Dr. Anthony S. Fauci, the Biden administration’s top Covid adviser, told reporters on Wednesday.
The C.D.C. has previously published data showing that second and third doses of the Moderna and Pfizer-BioNTech vaccines were less effective at fending off the Omicron variant than the Delta variant. Third doses enhanced that protection, at least in the short term.
In the new study, a team of researchers funded by the C.D.C. collected data from 10 states between Aug. 26, 2021, and Jan. 22, 2022. The data include periods during which the Delta and Omicron variants, respectively, accounted for more than 50 percent of cases in the country.
The team analyzed 241,204 visits to emergency departments or urgent care facilities and 93,408 hospitalizations among adults older than 18. The researchers did not include milder coronavirus infections.
Protection against emergency department and urgent care visits declined from 69 percent within two months of the second dose to 37 percent after five months or more. Booster shots restored those levels to 87 percent.
The effectiveness of boosters also waned. Protection against emergency department and urgent care visits dropped to 66 percent within four or five months, and to just 31 percent after five or more months of receiving the third shot, the researchers found. The latter estimate may not be reliable because few people received boosters more than five months ago, and so the data is limited, the researchers said.
But the protection provided by both second and third shots against hospitalization was generally higher than for emergency department and urgent care visits.