Does it help to screen for major diseases? Experts share pros, cons, and challenges
Screening for colorectal cancer (CRC) has proven very effective in diagnosing the disease and preventing deaths, according to epidemiologist Hermann Brenner.
Speaking at the 10th annual Cutter Symposium at Harvard T.H. Chan School of Public Health, held virtually May 15, Brenner, a professor from Heidelberg University, noted that there are two million CRC cases every year around the world—and one million deaths. “Most of those deaths would be preventable by effective screening,” he said.
The symposium, which focused on how well screening works for various diseases—and how it might work better—also featured Harvard Chan School’s Lorelei Mucci, who spoke about prostate cancer screening, and Stanford University’s Marc Lipsitch, who discussed screening for pandemic diseases.
The event was organized by the Department of Epidemiology and was moderated by department chair Albert Hofman, Stephen B. Kay Professor of Public Health and Clinical Epidemiology. Hofman noted that the Cutter Lectures and Symposia have been held since 1912, funded by a bequest from John Clarence Cutter, a graduate of Harvard Medical School.
Brenner, who is also scientific coordinator of the National Cancer Prevention Graduate School at the German Cancer Research Center in Heidelberg, outlined the three established methods for CRC screening—stool tests, flexible sigmoidoscopy (in which the lower half of the colon is examined), and colonoscopy (examination of the entire colon). All of the methods have been shown to help, he said. For instance, a 2013 study showed that people who got annual stool tests had a 30% reduction in colon cancer mortality, and a 2024 study found that colon cancer mortality was almost halved among those who had only one sigmoidoscopy. Colonoscopy is a more invasive procedure, usually done under sedation, and it involves an unpleasant prep—but it’s the most effective type of screening, Brenner said. A landmark 1993 study showed an approximately 90% reduction in incidence of colon cancer among those who had polyps removed during colonoscopies.
Brenner noted that the proportion of people in the U.S. getting colonoscopies has increased from 20% in 2000 to between 60% and 70% today. “At the same time, the incidence of colorectal cancer in the U.S. has gone down dramatically,” he said.
He noted, however, that the decline in CRC incidence has been mostly among people older than 50, whereas it has increased among people aged 20-49. While the U.S. has lowered its recommended screening age for CRC from 50 to 45, other countries have been more reluctant to do so, he said. In the absence of routine screening for younger people, he suggested that one strategy would be to offer screening to those considered at high risk for the disease.
PSA puzzle
While there’s broad consensus on the benefits of screening for CRC, the picture for prostate cancer screening is murkier, according to Mucci, professor of epidemiology.
Prostate cancer is one of the most commonly diagnosed cancers in the world, with more than 1.6 million new diagnoses and nearly 400,000 deaths each year. Some of the highest mortality rates are among Black men, Mucci said.
Prostate specific antigen, or PSA, has been used as a screening tool for prostate cancer since the mind-1980s, after it was shown that levels of the enzyme in the body may change in the presence of tumor cells. But measuring PSA is not a firm predictor of cancer; for instance, Mucci said, some tumors may not impact PSA levels at all. Further, screening can lead to overdiagnosis, resulting in treatment of cancers that likely wouldn’t cause harm. And treatments such as surgery and radiation carry risks such as urinary incontinence and sexual dysfunction.
In 2012, the U.S. Preventive Services Task Force—concerned about overdiagnosis and the risk of infection from biopsies—recommended against using PSA-based screening for prostate cancer. Later, a 2022 study that examined prostate cancer trends in the wake of the new guidelines found a large uptick in incidence rates of metastatic cancer, particularly among Black men.
Mucci thinks evidence makes clear that PSA screening does help reduce prostate cancer deaths. But she described the current landscape of prostate cancer screening guidelines as a “quagmire.” Recommendations vary widely among medical groups; for instance, the American Academy of Family Physicians doesn’t recommend routine PSA screening, whereas the American Urologic Association suggests that men get screened every 2-4 years beginning at age 45.
Mucci suggested possible ways to improve screening. One might be to measure men’s baseline PSA levels in midlife—since those levels have been shown to predict lethal prostate cancer risk—and use that information to guide screening decisions. Researchers might also consider taking individuals’ genetic risk into account.
A push for pandemic screening
Lipsitch, Berberian Professor and Senior Fellow at Stanford’s Center for International Security and Cooperation, a professor of medicine and biology at Stanford, and an adjunct professor of epidemiology at Harvard Chan School, said it’s extremely important to screen for pandemic diseases because it can provide experts with data to help shape better public health and medical decisions; shed light on the prevalence of disease and which populations are most at risk; and uncover people with asymptomatic infections, who can then take preventive measures to reduce their contact with others.
Lipsitch outlined five principles for effective screening during a pandemic:
Choose the right diagnostic tool for the job. The tool should be low-cost, have a rapid turnaround, and have the ability to be used globally and be acted on quickly.
Screen populations randomly for infection. Such screening was done in the U.K. during COVID, and it provided a helpful picture of the prevalence of infection, the appearance of new variants, and vaccine effectiveness.
Integrate multiple data sources. In addition to traditional health care data, experts could also consider data such as people’s digital trails (information gleaned from internet searches, wearables, or social media activity, for example) and wastewater sampling. Lipsitch noted, for instance, that during the COVID pandemic, wastewater sampling showed that SARS-CoV-2 levels dropped one to two weeks before hospitalizations did. Such information can help hospitals forecast admissions.
Update diagnostic tests so they can detect new variants of a pathogen. “The sensitivity and specificity of infectious disease diagnostics can change as the pathogens evolve,” Lipsitch said.
Link multiple types of data. Often, there are separate surveillance systems for demographic information, genomic information, vaccination data, and clinical data. Housing that data in one system would make it much more valuable, he said.
During the COVID pandemic, for example, when Lipsitch was working at the Center for Disease Control and Prevention’s Center for Forecasting and Outbreak Analytics, he and colleagues were able to examine electronic health records from the Kaiser Permanente Southern California system, vaccination data, and genomic information to show that the Omicron variant was not as dangerous as the Delta variant—it carried lower risk of hospital admission, mechanical ventilation, and death.
“But pulling all this together was good luck—cobbling together something that ideally one would not have to cobble together,” he said, noting that that experience suggests that a more comprehensive screening strategy would be the best way forward.
