A retrospective analysis of data from the Framingham Heart Study has determined that, although heavy smokers have a significantly decreased risk of cardiovascular disease (CVD) within five years after smoking cessation in comparison to current smokers, their risk remains significantly elevated compared to those who have never smoked. Writing in JAMA, Meredith S Duncan (Vanderbilt University Medical Center, Nashville, USA) et al conclude that the findings affirm the cardiovascular benefit from quitting smoking but reveal that CVD risk declines slowly over decades.
Current risk calculators consider former smokers to be at risk for only five years; Duncan and colleagues aimed to evaluate the association between years since quitting smoking and incident CVD. The observational cohort study of Framingham Heart Study participants analysed prospectively collected data of those without baseline CVD from original cohort members attending their fourth examination cycle (1954–58) and offspring cohort participants attending their first examination cycle (1971–75), when smoking data were first reliably collected.
Participants were categorised into current, former, or never smokers based on self-reported smoking habits at baseline. Years since quitting for former smokers, as well as cumulative pack–years of smoking for ever smokers, were calculated. The pooled analyses of both cohorts was restricted to heavy ever smokers (≥20 pack–years), with follow-up to December 2015 for development of CVD outcomes, including myocardial infarction, stroke, heart failure, and CVD death.
The study population consisted of 8,770 individuals (3,805 from the original cohort, and 4,965 from the offspring cohort), with a mean age of 42 years and 45% were male. There were 5,308 ever smokers with a median 17.2 baseline pack–years. This included 2,371 heavy ever smokers, of which 406 (17%) were former smokers and 1,965 (83%) were current smokers.
Median follow-up was 26.4 years, during which time 2,435 first CVD events occurred (1,612 in the original cohort, of which 665 were among heavy smokers, with 823 in in the offspring cohort, 430 among heavy smokers). In the pooled cohort, compared with current smoking, quitting within five years was associated with significantly lower rates of incident CVD (incidence rates per 1,000 person–years: current smoking 11.56, 95% confidence interval [CI] 10.30–12.98; quitting within five years 6.94, 95% CI 5.61–8.59; difference −4.51, 95% CI −5.90 to −2.77) and lower risk of incident CVD (hazard ratio [HR] 0.61, 95% CI 0.49–0.76).
In a comparison between former smoking and never smoking in the pooled cohort, quitting smoking ceased to be significantly associated with greater CVD risk between 10 and 15 years after cessation (incidence rates per 1,000 person–years: never smoking 5.09, 95% CI 4.52–5.74; quitting within 10 to <15 years 6.31, 95% CI 4.93–8.09; difference 1.27, 95% CI −0.10–3.05; HR 1.25, 95% CI 0.98–1.60).
The authors say that the findings are primarily “applicable to ever smokers with a cumulative smoking history of at least 20 pack–years”, and do not incorporate information on environmental tobacco smoke exposure. Additionally, as the data is from the Framingham Heart Study, composed primarily of white individuals of European ancestry, its generalisability may be limited.
However Duncan et al add: “This investigation extends prior knowledge by using a rigorous self-reported smoking ascertainment method that includes prospectively gathered and regularly time-updated data on smoking status and intensity collected during in-person visits. Compared with never smoking, it took 10 to 15 years (pooled cohort) (five to 10 years in the original cohort and ≥25 years in the offspring cohort) following cessation for former heavy smoking to cease being significantly associated with elevated CVD risk.” The authors further emphasise that now is a great time to quit smoking.