Flu occurs every year, why is it so severe this year?

As winter approaches and the temperature drops sharply, influenza viruses begin to become active. For a time, respiratory infection terms such as "influenza", "fever" and "mycoplasma" frequently appeared on hot searches.

Data from disease control monitoring in various regions show that multiple infections such as influenza, COVID-19, adenovirus, Mycoplasma pneumoniae, and respiratory syncytial virus are higher than in previous years. Not only children and the elderly, but also young and middle-aged people, who were previously considered to have the strongest immunity, are now "infected" without distinction.

Some experts have suggested that the frequent occurrence of respiratory infectious diseases may be caused by the human body's "immune gap" . Let's talk about this today.

01

Are the frequent occurrences of respiratory infectious diseases due to the “immunity gap”?

As early as 2021, immunologists proposed the concept of "immunity debt" [1], also known as the "immunity gap" , which refers to the lack of immune stimulation to a certain pathogen due to long-term low frequency or no contact with a certain virus or bacteria. The accumulation of this lack of immune system leads to a decline in herd immunity. Of course, the reduction in the circulation of pathogens in nature and the reduction in the vaccination rate of related vaccines are also important reasons.

"Immune debt" does not mean that the immune system is in debt, and "immune gap" does not mean that there is a big crack in the immune system. It is just a figurative expression. We can also call it "immune laziness." More precisely, it means that the immune system loses its memory resistance to specific pathogens after a long period of time without contact with specific pathogens. This is the same as professional athletes who do not train for a long time, their muscles will become lazy, and they will gradually lose strength and professional skills.

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On November 24, 2023, Maria Van Kerkhove, executive director of the World Health Organization's Epidemic Preparedness Department, said in an interview that the main reason for the current surge in respiratory infections among children in China is the immunity gap related to the COVID-19 pandemic.

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During the fight against the epidemic, we adopted scientific and effective health protection measures, including wearing masks, washing hands frequently, and popularizing vaccines. While effectively curbing the spread of the new coronavirus, it also played a certain role in preventing and controlling the spread of other pathogens, especially respiratory infectious diseases [2]. Studies have also shown that the positive detection rates of influenza, respiratory syncytial virus, and adenovirus have all decreased during the new coronavirus epidemic, and the number of cases has also been greatly reduced[3].

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After the introduction of normalized prevention and control policies, exchanges between countries and regions gradually returned to normal. With more exchanges and contacts between people, the incidence of pathogens, especially respiratory pathogens, rebounded.

In the decade before the outbreak, China conducted a nationwide prospective surveillance of acute respiratory infections of all ages [4]. The results showed that influenza virus, respiratory syncytial virus, and human rhinovirus were the three major viral pathogens infecting children under 5 years old and school-age children (accounting for 28.5%, 16.8%, and 16.7%, respectively), and Streptococcus pneumoniae, Mycoplasma pneumoniae, and Klebsiella pneumoniae were the three major bacterial pathogens (accounting for 29.9%, 18.6%, and 15.8%, respectively). Therefore, for children born after the outbreak or at a younger age, this "immunity gap" is more likely to delay the age at which they are first exposed to common pathogens.

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A similar view was put forward in an analysis published in the international journal Journal of Medical Virology by a professor from Peking Union Medical College . Since the influenza virus's own ability to spread is stable, it is reasonable to assume that the explosive increase in influenza infections is more likely to be due to the increased susceptibility of the population during the COVID-19 period.

02

What impact does the “immunity gap” have on individuals?

So what impact does this "immunity gap" have on everyone's immune system, especially children's? In other words, if an individual lacks immunity to a pathogen because they have not been exposed to it for a long time, will they experience more severe symptoms when they are infected with this pathogen again?

In fact, every normal person's body has a complete immune system, which can identify the body's own tissues, foreign tissues, and toxic and harmful substances, thereby preventing pathogens from invading the human body and effectively ensuring good health.

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The first immune barrier comes from the barrier of our skin and respiratory mucosa , which can prevent pathogens and other microorganisms from entering the human body in large quantities; the second comes from non-specific immune cells , which will identify all foreign "foreign bodies" and remove these "foreign bodies" by engulfing and stimulating inflammatory reactions in body cells. The third comes from specific immune cells, which are "memory" and produce "immune warriors" with "battle memory" for specific pathogens through the experience of previous infection with pathogens, and specifically prevent specific pathogen infection. If there is a lack of fighting with a certain pathogen for a long time, our "immune warriors" will be dormant, and "immune warriors" with battle memory will even "retire". In the case of long-term lack of exercise, once faced with a large number of "pathogen enemies" at once, there will definitely be a situation of being overwhelmed. Therefore, the "factory" of the immune system is not irreversibly damaged or incapable of working, but there is a lack of "inventory" in the short term and the "production capacity" is insufficient. When normal exercise is resumed and various "battle training" begins again, the abilities of various "immune warriors" will be brought into play, and immune barriers at all levels will be gradually established.

03

How to offset the impact of the “immunity gap”?

If our bodies have already developed an "immunity gap", how can we offset or reduce its impact? First, we should maintain a healthy lifestyle and a positive attitude, exercise appropriately, strengthen dietary nutrition, develop regular work and rest habits, ensure adequate sleep, and let the body get enough rest. In addition, try to avoid being in an unventilated closed environment for a long time. Even in the cold winter, it is recommended to open windows for ventilation.

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Then, pay attention to personal hygiene and develop good hand-washing habits. During the epidemic season, you should still wear a mask when entering public places or crowded places. If you are unfortunately "infected", there is no need to worry. Strengthen rest and various nutritional supplements, drink more water, and most people can "heal on their own". A small number of serious cases require timely medical treatment and use antipyretic and anti-pathogen drugs according to the doctor's advice. In this case, it is recommended to wear a mask to avoid cross-infection with people around you.

Finally, for the elderly, young children, people with weak constitutions and those with underlying diseases, it is recommended to receive relevant vaccines in a timely manner before the epidemic season every year. Vaccines can awaken "immune warriors" with combat memories and help the human body quickly establish a specific immune barrier.

References

[1] Cohen R, Ashman M, Taha MK, et al. Pediatric Infectious Disease Group (CPIP) position paper on the immune debt of the COVID-19 pandemic in childhood, how can we fill the immunity Gap? [J]. Infect Dis Now, 2021, 51(5): 418-423.

[2] Yang DD, Ouldali N, Gajdos V, et al. Common Pediatric Respiratory Infectious Diseases as Possible Early Predictor for New Wave of Severe Acute Respiratory Syndrome Coronavirus 2 Infections[ J]. Clin Infect Dis, 2021, 73(2): 358-359.

[3] Jiang Chenyan, Cui Peng, Pan Hao, et al. Impact of novel coronavirus pneumonia prevention and control measures on influenza transmission[J]. Shanghai Preventive Medicine, 2021, 33(12): 1109-1112,1175.

[4] Li ZJ, Zhang HY, Ren LL, et al. Chinese Centers for Disease Control and Prevention (CDC) Etiology of Respiratory Infection Surveillance Study Team. Etiological and epidemiological features of acute respiratory infections in China. Nat Commun. 2021 Aug 18;12(1):5026. doi: 10.1038/s41467-021-25120-6. PMID: 34408158; PMCID: PMC8373954.

Author: Shi Zhaowen, deputy chief physician, Shanghai Pudong District Central Hospital Editor: Yiren Bai