For cases for which it was unknown, the date of illness onset was estimated by subtracting 4 days (median interval between onset of symptoms and date of specimen collection for cases in this data whose date was known) from the first sample collection.
Katika visa vilivyokosa tarehe ya kuanza, tarehe ya kuanza ilikadiriwa kwa kuondoa siku 4 (muda wa kati kutoka kuanza kwa dalili hadi kukusanywa kwa data miongoni kwa visa vilivyo na tarehe zinazojulikana kwenye data hizi) kutoka wakati mapema zaidi sampuli ilipokusanywa.