We thank Nauta  for providing further explanation on the need to control calendar time in TNDs. We agree that such adjustment is probably redundant in temperate climates with well-defined influenza seasons and well-defined influenza vaccination campaigns. However, we caution those researchers conducting TND studies in tropical and sub-tropical regions where there may be multiple influenza seasons. A vaccination campaign scheduled for April or September, when the Southern and Northern hemisphere vaccines are typically available, may fall before, during or even after the peak of the influenza season. When the campaign and influenza season coincide, vaccination status would not be in steady state and the need to adjust for calendar time persists.