Objective:To summarize the clinical features, viral load changes, and outcomes of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) Omicron variant infection in mother-infant dyads during lactation period.Methods:A total of 24 pairs of lactating mothers and infants under one year old who were infected with SARS-CoV-2 and hospitalized in Lingang Branch of Shanghai Sixth People's Hospital from April 8 to May 30, 2022, were selected as the lactation group in this retrospective study. Another 24 non-lactating mothers, with children of one to three years old, who matched with those mothers in the lactation group in clinical classification and admission date were selected as the control group. Vaccination status, clinical symptoms, daily cycle threshold (Ct) of open reading frame 1ab ( ORF1ab) gene and nucleocapsid protein ( N) gene, and the duration of positive nucleic acid test were compared between the groups and were analyzed using two independent samples t test, one-way analysis of variance, LSD test, and Chi-square test. Results:Among the 24 infants in the lactation group with an age of (6.5±2.1) months, 23 cases were mild type, one was common, and none had been vaccinated against SARS-CoV-2. The maternal age of the lactation and the control group did not differ statistically [(28.7±6.4) vs (28.2±5.2) years, t=0.30, P=0.768]. Mothers with mild type accounted for 88% (21/24) and those with common for 12% (3/24) in both groups of mothers. Three mothers received one dose of vaccine and two received two in the lactation group, while three received one dose and three received two in the control group [21%(5/24) vs 25%(6/24), χ 2=0.12, P=0.731]. The most common symptoms of lactating infants were fever (100%, 24/24) , followed by diarrhea (58%, 14/24) , cough (50%, 12/24), and wheeze (29%, 7/24), those of the lactating mothers were fever (75%, 18/24) , cough (75%, 18/24) , and sore throat (63%, 15/24) , while those of non-lactating mothers were cough (88%, 21/24) , sore throat (71%, 17/24), and fever (58%, 14/24). The duration of positive nucleic acid test was the shortest in the lactating infants [(9.2±2.1) d (5-14 d)], followed by mothers in the control group [(11.2±2.4) d (6-16 d)] and mothers in the lactation group [(14.0±4.2) d (8-26 d)] (LSD test, all P<0.05). Each day from day 2 to 9 after diagnosis, Ct values of nucleic acid of infants in the lactation group were all higher than those of mothers in both the lactation and control groups (LSD test, all P<0.05). On day 10, Ct value of nucleic acid infants was higher than that in mothers in the lactation group ( ORF1ab gene: 37.91±4.34 vs 32.79±5.47; N gene: 37.95±4.58 vs 32.66±5.77), which was lower than those in mothers in the control group ( ORF1ab gene: 32.79±5.47 vs 35.90±4.17; N gene: 32.66±5.77 vs 36.08±4.16) (LSD test, all P<0.05). On day 11, the nucleic acid Ct values of mothers in the lactation group were all lower than those in the control group ( ORF1ab gene: 35.03±3.74 vs 37.84±3.26, t=－2.78, P=0.008; N gene: 35.30±3.75 vs 38.11±2.90, t=－2.90, P=0.006). On day 12, Ct value of ORF1ab gene and N gene in mothers in the lactation group were similar to those in mothers in the control group (both P>0.05). Conclusions:The SARS-CoV-2 vaccination rate of mothers and infants were low during lactation. Lactating infants infected with SARS-CoV-2 Omicron variant have low virus load and may have a quick recovery, while for the lactating mothers, the virus load is high and the recovery is slow.