The purpose of this study is to examine the incidence of acute myocardial infarction (MI) occurring after an influenza-like illness using linked primary care and disease registry databases.

This study will measure the incidence ratio for MI occurring in time periods after presentation with an acute respiratory illness compared to baseline time periods using the self-controlled case series method, with reference to the timing of the influenza season (defined by national virological and clinical surveillance data). It will also validate accuracy and completeness of information on MI in the GPRD using linked anonymised data from both MINAP and Hospital Episode Statistics.

This study is part of the CALIBER (Cardiovascular disease research using linked bespoke studies and electronic records) programme funded over 5 years from the NIHR and Wellcome Trust. The central theme of the CALIBER research is linkage of the Myocardial Ischaemia National Audit Project (MINAP) with primary care (GPRD) and other resources. The overarching aim of CALIBER is to better understand the aetiology and prognosis of specific coronary phenotypes across a range of causal domains, particularly where electronic records provide a contribution beyond traditional studies. CALIBER has received both Ethics approval (ref 09/H0810/16) and ECC approval (ref ECC 2-06(b)/2009 CALIBER dataset).

Inclusion Criteria:

• Participants must be registered with a GPRD 'up to standard' practice during the study period 01/01/1999-31/12/2008
• They must have a record of an incident myocardial infarction (defined by a Read codelist submitted to and agreed by GPRD) that occurs both within their registration and the study period
• They must have at least 6 months between their registration date and the date of the incident MI
• They must have accessed primary care on at least one occasion for a respiratory illness during the study period

Exclusion Criteria:

• Not fulfilling above criteria

Patients with an incident myocardial infarction who also have a record of a visit to primary care with a respiratory tract infection occurring within the study period (01/01/99 - 31/12/2008)