Influenza virus is a member of the Orthomyxoviridae family, with N clones, and most clones can undergo "seventy-two transformations", making it difficult for humans to prevent.
1. Typing
Type A influenza is the most common and can not only infect humans, but also various animals such as wild birds, poultry and livestock, marine mammals, etc. Therefore, according to animal hosts, it is also divided into avian influenza, swine influenza, equine influenza, etc. The subtypes of influenza A virus are diverse and determined by two surface proteins: hemagglutinin (HA) and neuraminidase (NA). At present, 18 subtypes of HA and 11 subtypes of NA have been discovered, and different combinations of HA and NA constitute different virus subtypes, although not all combinations can infect humans. The influenza A virus poses the greatest threat to humans, not only causing seasonal epidemics every year, but also all influenza pandemics in human history are caused by new variants of the influenza A virus.
Type B influenza is also quite common, with a relatively single host. Currently, only type B influenza virus has been found in humans and seals. The influenza B virus is divided into two lineages, B/Victoria and B/Yamagata. Its symptoms are similar to those of influenza A. These two lineages were formed in the 1980s and have since alternated as seasonal influenza. However, since 2020, the B/Victoria lineage of influenza B has been prevalent in the population, and B/Yamagata has not been prevalent again.
The C influenza virus can infect humans and has been found in animals such as pigs, dogs, and cows. The C influenza virus has no subtype classification and usually causes mild respiratory infections. It mostly appears in children and rarely triggers large-scale epidemics. Overall, influenza C poses less of a serious threat to human health.
2. Epidemic characteristics
The peak of seasonal influenza outbreaks in China varies between the north and south. In the north, it is usually the winter and spring seasons, while in the south, in addition to the winter and spring seasons, there is often a high incidence of influenza outbreaks in summer. The entire population is generally susceptible. During the annual flu season, the incidence of influenza in children ranges from 20% to 30%, and in some high prevalence seasons, the annual infection rate can reach up to 50%. And it is prone to mutation, which makes it difficult to maintain long-term immune protection formed by natural infection or immunization.
3. Symptoms after infection
After being infected, the early main symptoms for most people are fever, with a body temperature of up to 39-40 ° C, accompanied by chills and shivering; Often accompanied by systemic symptoms such as headache, muscle soreness, fatigue, and decreased appetite; Cough, sore throat, runny nose or nasal congestion, nausea, vomiting, diarrhea and other symptoms often occur.
High risk populations such as infants, pregnant women, elderly people, and chronic disease patients have a relatively high risk of developing serious complications when infected. The condition of critically ill patients develops rapidly, and their body temperature often remains above 39 ° C. They can rapidly progress to ARDS (acute respiratory distress syndrome), sepsis, septic shock, heart failure, renal failure, and even multiple organ dysfunction.
4. Transmission speed
The incubation period is usually 1 to 3 days, and it is contagious from the end of the incubation period to the recovery period. The replication speed is fast, usually reaching its peak 24 hours after infection with the virus. It is mainly transmitted through respiratory droplets produced by sneezing and coughing, and can also be transmitted through direct or indirect contact with mucous membranes such as the mouth, nose, and eyes, such as touching contaminated items with hands and touching one's own mouth and nose. In addition, there are a small number of asymptomatic infected individuals who are already infected but are not aware of it, which may lead to their transmission silently.
5. Response strategy
Currently, vaccination is the most economical and effective way to prevent infection. It is recommended to receive the flu vaccine in a timely manner before the flu season each year (such as before October); Those who have not been vaccinated in a timely manner can receive re vaccination throughout the entire flu season. All individuals aged 6 months and above with no contraindications for vaccination can receive the influenza vaccine, especially high-risk groups such as the elderly, children, and patients with chronic underlying diseases.