In a recent article, Aleck et al. suggested that a body of previous data reported by Aaskov et al. could be attributed to “laboratory or other error.” The specific issue was the observation by us that three children of the 11 born to Ross River virus-infected mothers had anti-Ross River virus IgM in their circulation at birth although the mothers did not. One of the three IgM-negative mothers had no detectable hemagglutination-inhibiting (HI) antibody against Ross River virus at a dilution of 1:20.

In adults, IgM antibody against Ross River virus is present for about 10 weeks post onset of symptoms (range 3–35 weeks) but absent after about 19 weeks (range 7–44). We are unaware of any similar data for IgM persistence following fetal infection. Since the mothers were infected between 11 and 19 weeks post-conception it would not be surprising if some of them were IgM negative 21–29 weeks later.