Americans who received the single-dose Johnson & Johnson Covid vaccine were better protected against severe illness and hospitalization during the Omicron surge if they received a second dose of the Pfizer-BioNTech or Moderna vaccines, instead of an additional Johnson & Johnson shot, according to a new study published on Tuesday by the Centers for Disease Control and Prevention.

The report did not offer a comparison with the protection afforded by two doses of mRNA vaccines and included relatively few Johnson & Johnson recipients, making the findings difficult to interpret.

The findings broadly support the added benefit of a booster dose against the Omicron variant, which is known to partially sidestep immune defenses. Yet the report is somewhat at odds with other data collected by the C.D.C. that suggest that a single dose of the Johnson & Johnson vaccine prevented infections with the Omicron variant at least as well — if not better — than two doses of the mRNA vaccines.

Those data do indicate that people who received a single dose of the Johnson & Johnson vaccine are at slightly higher risk of death than those who received two mRNA doses.

Separately, South African researchers have found that two doses of the Johnson & Johnson vaccine offer protection against severe illness and hospitalization that is comparable to that seen with two doses of the Pfizer-BioNTech vaccine.

The C.D.C. now recommends that all adults who received one or two doses of the Johnson & Johnson vaccine get a booster dose of an mRNA vaccine. On Tuesday, the Food and Drug Administration authorized a second booster shot for all adults 50 and older, even those who already have had three mRNA doses (that is, two for full vaccination plus one booster).

In the new study, researchers analyzed data from 80,287 Covid-related visits to emergency departments or urgent care clinics and 25,244 hospitalizations in 10 states. The data were tallied from Dec. 16, 2021 to March 7, 2022, when the Omicron variant was the predominant version of the virus.

A single dose of the Johnson & Johnson vaccine had an effectiveness of 24 percent for preventing E.R. and urgent care visits, compared with 54 percent after two doses of the vaccine.

The effectiveness of a single dose of the Johnson & Johnson vaccine combined with a booster shot of an mRNA vaccine was similar to the protection from three mRNA doses, the study found. (This finding is consistent with the results from other studies of booster shots.)

Vaccine effectiveness against hospitalization showed a similar trend: 31 percent for a single dose of Johnson & Johnson, 67 percent for two doses of the vaccine, 78 percent for one dose of the Johnson & Johnson vaccine plus one of an mRNA vaccine, and 90 percent after three doses of an mRNA vaccine.

But the margins of error for these estimates overlapped, meaning that the differences may not be meaningful.

The researchers noted that the study has other limitations. The data do not include data beyond two months after receipt of the last dose on average. Other studies have suggested that the mRNA vaccines wane sharply in effectiveness against infection after an initial peak, while the Johnson & Johnson vaccine is performing better long-term.

The study included less data regarding Johnson & Johnson recipients than about those who got mRNA vaccines, making the comparisons less reliable. For example, the researchers recorded 164 hospitalizations among people who received two doses of the Johnson & Johnson vaccine, compared with nearly 8,000 among recipients of three mRNA doses.

And the numbers overall were too small to parse vaccine recipients by age, sex or presence of other health conditions, all of which may have skewed the results.