John W. Rowe is a professor of health policy and aging at Columbia University and president of the International Association of Gerontology and Geriatrics.

The past three months have been a stark reminder that the United States is an aging society. Americans older than 60 outnumber those younger than 15. And this rapidly expanding older population is being devastated by the novel coronavirus.

But we have also learned a great deal since March about why seniors are so susceptible to this virus — and which symptoms to look for when they present themselves at clinics and hospitals looking for help.

First, a few things to remember. About 80 percent of covid-19-related deaths are in individuals older than 65. In New York City, for hospitalized patients the death rate from covid-19 rises from less than 1 percent for those 18 to 44 years old to more than 30 percent for those older than 75.

These findings reflect what we already knew about how the body’s response to infection changes with age.

With normal aging, we see progressive deleterious changes in our immune systems. One component, inflammation, is exaggerated with advancing age — a finding termed “ inflamm-aging” — which can worsen infections when we get them. Other components of our immune systems weaken with age because the cells that fight off invading organisms directly, or through release of antibodies, become less effective in old age. That’s why older people need stronger doses of the flu vaccine to elicit a protective response.

Another reason that aging complicates matters is that many diseases, and pneumonia in particular, have very different symptoms in the elderly than they do in the middle-aged. Many older covid-19 patients did not present the “classic” symptoms so common in younger patients — fever, cough and shortness of breath. Seniors who came to hospitals or called their physicians were typically asked whether they had those symptoms and often replied that they did not.

But many did complain of delirium, fainting or gastrointestinal symptoms. Because they did not have the symptoms that were required to be eligible for coronavirus testing, their infections often went undetected and were left to worsen without appropriate attention.

These age-related differences are aggravated by the increased risk associated with severe underlying illnesses, such as diabetes; heart failure; lung, kidney or liver diseases; or cancer, especially those on treatments that might impair immune function. While it is likely a healthy 75-year-old is at less risk than a 60-year-old with a chronic disease, a 75-year-old with a chronic disease is at special risk.

In addition, many elderly people share the same disadvantageous social determinants of health that contribute to the special risks of populations, including the poor, who might have substandard housing, nutrition and access to health care.

Beyond these factors, several additional risk factors for severe covid-19 have been identified, and more are sure to come:

· Men are more susceptible than women.

· Obesity increases risk, at least in young and middle-aged adults (though not so much in seniors).

· Having Type-A blood carries a 50 percent greater risk of requiring respiratory support (such as a ventilator) when infected, while Type-O blood lessens that risk. (If you don’t know your blood type — and many do not — it is time to find out.)

· And a genetic marker, called ApoE4, which increases the risk of Alzheimer’s, also increases the risk of severe covid-19. This makes individuals with dementia an ultra-high-risk group, especially given their difficulty in following guidelines such as social distancing and frequent hand-washing.

Understanding an individual’s risk profile might become useful in allocating a vaccine when it becomes available. One can imagine the competing interests of health-care providers, chronically ill elderly people, and public health and public safety officers when the initial batches of vaccine become available.

What can we do to protect older people, especially those at greatest risk?

We must heighten our detection of coronavirus infection through increased testing and education of health-care providers, particularly at nursing homes, regarding the signs and symptoms older patients might display.

All older people must receive the flu vaccine this fall, for if covid-19 returns during a strong flu season, the effects might be devastating.

While we enhance nursing home safety, disinfection and protection of staff — all of which have been accomplished successfully at many facilities — the time has come to rethink how we deliver and fund long-term care in the United States. And we must fix our public health system, which has never been retooled to meet the needs of an aging society and is displaying deficiencies in pandemic preparedness.

Neither our current long-term-care nor our public health system can adequately support the needs of a dramatically growing older population or meet the special challenges presented by this, and the next, pandemic.

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