Which, in the article is followed by this statement:
There were no appreciable between-group differences in the time to PCR conversion or culture conversion according to vaccination status,
Context matters.
You would do better to include the entire paragraph, and then you might see that the "groups" they are referring to are groups based on covid strain (ie: delta vs omicron), not vaccination status:
The characteristics of the participants were similar in the two variant groups except that more participants with omicron infection had received a booster vaccine than had those with delta infection (35% vs. 3%) (Tables S1 and S2 in the
Supplementary Appendix, available with the full text of this letter at NEJM.org). In an analysis in which a Cox proportional-hazards model that adjusted for age, sex, and vaccination status was used, the number of days from an initial positive polymerase-chain-reaction (PCR) assay to a negative PCR assay (adjusted hazard ratio, 0.61; 95% confidence interval [CI], 0.33 to 1.15) and the number of days from an initial positive PCR assay to culture conversion (adjusted hazard ratio, 0.77; 95% CI, 0.44 to 1.37) were similar in the two variant groups (
Figure 1A through 1C and S1 through S3, and Tables S3 through S5). The median time from the initial positive PCR assay to culture conversion was 4 days (interquartile range, 3 to 5)
in the delta group and 5 days (interquartile range, 3 to 9)
in the omicron group; the median time from symptom onset or the initial positive PCR assay, whichever was earlier, to culture conversion was 6 days (interquartile range, 4 to 7) and 8 days (interquartile range, 5 to 10), respectively. There were no appreciable between-
group differences in the time to PCR conversion or culture conversion according to vaccination status, although the sample size was quite small, which led to imprecision in the estimates (Figure 1D and 1E)..
