Influenza A(H1N1)2009 and vascular thrombosis - report of a case seriesArchived

Pandemic H1N1 Influenza Infection and Vascular Thrombosis Bunce PE, High SM, Nadjafi M, et al.  Clin Infect Dis. (2011) 52(2): e14-e17 

This retrospective cohort study, carried out in two hospitals in Toronto , Canada, looked for thrombotic events in all hospitalised patients with confirmed pandemic A(H1N1)2009 influenza. It then went on to describe the nature of these vascular events and the associated factors.  The publication has single illustrative a case report and a case series with a retrospective cohort study. The case report is of a patient who developed intravascular thrombosis during acute infection with 2009 pandemic influenza. The patient was a 50 year-old woman with confirmed 2009 A(H1N1) pandemic influenza and no significant previous medical history. She was admitted to hospital with a 1-day history of fever and respiratory symptoms. She developed respiratory distress and was found to have also suffered bilateral femoral artery thrombosis despite being given conventional anti-coagulant treatment for deep vein thrombosis prophylaxis. She underwent an urgent embolectomy combined with bilateral femoral artery stent placement in addition to systemic anticoagulation with intravenous un-fractionated heparin. After this procedure she gradually recovered from her infection and vascular complications.The retrospective cohort study had a study period from April to December 2009. The setting was two University of Toronto-affiliated tertiary care academic hospitals. Patient demographic information was recorded, as well as all vascular events, including clinical notes and diagnostic test reports. Vascular events were defined as thrombotic or embolic events occurring within the venous or arterial circulation. The results show that of 119 individuals admitted to hospital with confirmed pandemic infection 7 had confirmed thrombotic vascular events diagnosed, 4 of which were venous and 3 of which were arterial in origin. All patients with thrombotic events received Deep Vein Thrombosis (DVT) prophylaxis with subcutaneous un-fractionated heparin. None of the patients with thrombotic events were pregnant. Though there was no difference in age or sex between patients with and without thrombosis, but a non-significant trend towards increased mortality in patients with thrombotic events was observed, the mortality rate being 31% among patients with thromboses compared to 8% among patients without thromboses. Similarly, the mean duration of hospitalization was significantly longer for patients with thromboses than for patients without thromboses (36 days vs. 9 days; P < .001). In conclusion, the authors suggest that the presence of significant arterial thrombus formation in several of their patients points to an additional or alternative mechanism beyond stasis and hypercoagulability that may result in systemic endothelial activation during influenza A(H1N1)2009 infection, though they also stress the difficulty of inferring a link between influenza A(H1N1)2009 and the development of vascular thrombi on the basis of incidence rates alone.

ECDC Comment (28th January 2011):

With the predominance of A(H1N1)2009 in Europe this influenza season studies like this are becoming more important. As the authors state there have been few studies on this. However the rates of thrombotic episodes observed here is not that different from other case series with seasonal influenza. Also it is observed in one authoritative systematic review that seasonal influenza is associated with an increased incidence of acute cerebrovascular and cardio vascular events.(1)

1. Warren-Gash C, Smeeth L, Hayward AC Influenza as a trigger for acute myocardial infarction or death from cardiovascular disease: a systematic review Lancet Infectious Diseases, 2009; . 9 (10) 601-610. Read the article abstract