"The flu season seems to be arriving earlier every year. My grandchild in kindergarten has already caught it. I should get my flu shot early this year too."
Kim, 69, who had stopped by the neighborhood senior center for the first time in a while, found themselves in a quandary after discussing flu vaccinations with a peer. Kim had planned to get the free shot offered at the local public health center, as in previous years. But the acquaintance mentioned paying out of pocket for a "flu vaccine designed for older adults." Kim vaguely recalled hearing that immune responses to standard vaccines can weaken with age, prompting the development of vaccines with enhanced protection. Still, it was hard to tell whether the difference in effectiveness justified the extra cost.
Influenza may look like a common cold at first glance, but it is an entirely different illness. A cold is an upper respiratory infection caused by various viruses, including rhinovirus and adenovirus, with symptoms such as a runny or stuffy nose, coughing, sore throat and mild fever. Influenza, by contrast, is caused by influenza viruses and brings pronounced systemic symptoms — sudden high fever, chills, muscle aches and severe fatigue. It also carries a higher risk of secondary complications such as pneumonia and of hospitalization than a cold, and unlike a cold, it has both vaccines and antiviral treatments.
Influenza viruses change constantly and the circulating strains can shift, which is why annual vaccination is needed. Based on surveillance data from around the world, the World Health Organization each year forecasts and publishes which influenza strains are likely to circulate in the coming season in the Northern and Southern hemispheres. South Korea classifies adults aged 65 and over, pregnant women, and infants and children aged 6 months to 14 years as high-risk groups, and covers the cost of standard vaccines each year through the National Immunization Program (NIP). Products differ in some respects — whether the virus is grown in fertilized chicken eggs or in animal cells rather than eggs, and whether the maker is a domestic company — but the standard vaccines included in the NIP are considered to show no clinically meaningful differences in effectiveness. That is because vaccine makers worldwide receive the strains the WHO predicts are most likely to circulate that year and then culture and produce them, and antigen content standards are also fixed. As was the case last year, this year's NIP includes a trivalent flu vaccine protecting against two influenza A strains and one B strain, with vaccinations beginning on the 21st of this month for children and pregnant women in phased order. Older adults can receive the state-funded shot free of charge starting Nov. 12, according to a schedule based on age. But as high-immunogenicity flu vaccines aimed at older adults and others with weakened immunity have been introduced one after another, the number of people facing Kim's dilemma has risen sharply.
Behind older adults' vulnerability to influenza lies immunosenescence. Immunosenescence is the process by which immune function declines and regulatory capacity deteriorates with age. Degeneration of the thymus and bone marrow reduces the production and activity of immune cells, weakening the body's ability to detect and respond to invading viruses. The absence of typical symptoms such as high fever and severe muscle aches when older adults catch the flu is also closely tied to immunosenescence. Instead of a high fever, they may lose appetite, experience a sharp drop in energy or simply seem more sluggish than usual — which tends to delay medical care. The antibody response after vaccination is also neither as fast nor as strong as in younger years. "Immunosenescence progresses continuously, but the risk of infectious disease generally tends to rise noticeably after age 50," said Kim Jung-ho, a professor of infectious diseases at Severance Hospital. "Past 65, people are vulnerable to the flu, and because immunosenescence also reduces vaccine effectiveness, they are far more likely than younger people to progress to severe illness even when exposed to the same virus." The risk grows further for those who also have chronic conditions such as diabetes, chronic obstructive pulmonary disease (COPD) or cardiovascular disease. Data from the Korea Disease Control and Prevention Agency show that adults aged 65 and over account for more than 80% of flu-related hospitalizations. That is why countries around the world recommend giving high-immunogenicity flu vaccines first to people aged 65 and over.
High-immunogenicity vaccine is an umbrella term for flu vaccines designed to induce a stronger immune response than standard-dose vaccines. The category includes high-dose vaccines with increased antigen content, adjuvanted vaccines that add an immune-boosting agent, and recombinant vaccines that produce antigens using genetic recombination technology. In South Korea, two types have been introduced: a high-dose vaccine containing four times the antigen content per viral strain of standard vaccines, and an adjuvanted vaccine that adds an immune-boosting agent to help immune cells recognize antigens more effectively. "Whereas standard vaccines induce antibody formation through antigens, adjuvanted vaccines are designed to amplify the antibody response by adding an adjuvant such as MF59," Kim said. "That can help make up for defenses weakened by immunosenescence." Even among those 65 and over, the additional benefit from a vaccine varies with individual health status. People in their 80s and beyond with multiple chronic conditions, residents of long-term care facilities, cancer patients undergoing chemotherapy, recipients of organ or hematopoietic stem cell transplants and patients taking immunomodulatory drugs face a higher risk of severe illness from influenza than healthy people in their 60s, making high-immunogenicity vaccines worth considering first. One barrier to access, however, is that these vaccines are not covered by state support, leaving patients to pay the full cost themselves.
Regardless of type, flu vaccines take about two weeks after administration for protective antibodies to form. Getting vaccinated at the right time matters as much as deliberating over which vaccine to choose. Finishing before the end of October is best, but those who miss that window are better off getting the shot late than not at all. "You can still catch the flu after vaccination, but you can expect it to lower the risk of severe outcomes such as pneumonia and hospitalization," Kim said. "People aged 65 and over, especially those with chronic conditions, should complete their vaccination before the season takes hold."
