Transmission of Swine H1N1 In Japan Signals Phase 6

Commentary

Recombinomics Commentary 14:46
May 18, 2009

Japan's four earlier suspected swine flu cases came from travelers inbound from North America, but this new bout appears to be wholly domestic. Most of the cases involve teenagers - the first confirmed infected on Saturday were high-school students on a volleyball team - but those sick with the new strain of flu now range from as young as 5 years old to 60. "It circulated silently without anybody thinking of it," says Peter Cordingley, spokesman for the WHO in Manila. "The virus is highly transmittable and signs of it breaking out of a contained area into the greater community must be watched and taken very seriously."

The above comments on community transmission of H1N1 in Japan (see updated map) again indicate that the pandemic is at phase 6. Early definitions of pandemic phases used sustained transmission as the definition of phase 6. However, the new definition used sustained transmission as the definition of phase 4, while phase 5 represent transmission in a region, and phase 6 was transmission worldwide.

Swine H1N1 has been transmitting worldwide for over a month. Testing outside of North America was largely limited to travelers from North America, but H1N1 was seeding prior to the enhance surveillance at airports, and transmission would have been largely undetected.

The levels in the United States (see updated map) are approaching the reported levels in Mexico, and travel from the US would increase seeding. However, these increases would also be prior to enhanced surveillance at airports. However, such testing is destined to fail, because travelers infected within a day or two of travel would not be detected. Similarly, as many as 1/3 of cases have no fever.

Although the CDC estimated that the number of true cases in the US was 100,00, that estimate is likely to be 1-2 orders of magnitude too low. Surveillance in the US detects about 0.1% of true cases, so the 1500 cases in the CDC weekly report would represent 1.5 million cases in the US alone, and this estimate may be low because many cases are mild, and those in regions not reported to have high levels are not tested.

Nationwide, the number of confirmed cases of swine H1N1 (including cases that are influenza A positive and non-typable) is close to the level reported for H3N2 for the entire season. The number of swine H1N1 cases is rapidly increasing, and in week 18 was almost 3 times the level of influenza A (1097 vs 401).

Thus, the widespread transmission in North America (Mexico, US, and Canada) insures worldwide transmission. Community transmission has already been reported in Scotland, and now there is confirmed transmission in Japan, leaving no doubt that the spread in North America has been extended to Europe and Asia.

The raising of the pandemic level to 6 is long overdue.