> Reanalysis of the H1N1 sequences excluding isolates with unrealistic sampling dates indicates that the 1977 re-emergent lineage was circulating for approximately one year before detection, making it difficult to determine the geographic source of reintroduction. We suggest that a new method is needed to account for viral isolates with unrealistic sampling dates.
https://pmc.ncbi.nlm.nih.gov/articles/PMC2887442/
I will have to have a discussion with the editors of that page.