H1N1 swine flu is spreading fast enough to justify the preparations for a pandemic, say epidemiologists who’ve analysed the pattern of spread so far.

The reproductive number or R0 in Mexico in February was between 1.4 and 1.6 (each person who contracts flu passes it on to between 1.4 and 1.6 other people), approaching previous flu epidemics in 1918, 1957 and 1968.

However, there’s more uncertainty about how lethal the virus is, because there have been relatively few deaths so far.