Why do heat-afraid influenza viruses wreak havoc in summer?

The new coronavirus epidemic is not over yet, and influenza is here to add to the chaos.

Recently, the national influenza monitoring information released by the National Influenza Center showed that since June, some southern provinces have entered the peak influenza season, and the number of influenza cases has reached a record high in recent years.

Normally, influenza is most prevalent in winter and spring. Why is there an anti-seasonal peak in many southern provinces this year? How can we effectively prevent it? Science and Technology Daily interviewed relevant experts on this.

Flu is not only prevalent in winter and spring

Flu, short for influenza, is a highly contagious respiratory disease caused by influenza virus. In general, influenza virus seems to be very "afraid" of heat and usually only spreads in winter and spring, but this year it is "active" in summer, which makes many people confused.

In this regard, Guo Wei, deputy chief physician of the respiratory department of Tianjin Children's Hospital, said in an interview with Science and Technology Daily that summer flu is not unique to this year. The Spanish flu in 1918 broke out in May and also affected Guangzhou, Shanghai, Wenzhou and other places in my country.

"The reason why influenza is prevalent in many parts of southern my country this year is mainly related to the relatively low temperatures in these places," Guo Wei analyzed. For example, in May of previous years, the temperature in Fuzhou, Xiamen and other places could reach around 33 degrees Celsius, but this year, the temperature was only around 20 degrees Celsius, and the lowest temperature was even 13 degrees Celsius. In addition, due to continuous rainfall in many parts of the south, the temperature also dropped. Such temperature conditions can prolong the time for the influenza virus to maintain its infectiousness, and also create a suitable environment for its large-scale spread.

Chen Chunqi, a professor at the School of Life Sciences of Hubei University, further explained in an interview with reporters that in a high temperature environment, the flu virus is unstable and easily loses its activity, and its ability to spread will also decrease. "Generally, when the temperature exceeds 40 degrees Celsius, the flu virus's ability to spread is very weak; but in a low temperature environment of 4 degrees Celsius, the flu virus may survive for up to a month," she said.

With the high incidence of influenza in summer, many elderly people and children, especially preschoolers, have been "infected". "People who are susceptible to influenza are mainly those with low immunity, such as the elderly over 60 years old and preschoolers, as well as pregnant women, patients with chronic diseases and obese people with a BMI (body mass index) greater than 30," said Guo Wei. If you are under great work pressure or overworked, you are also likely to be "targeted" by the influenza virus.

Influenza A is more likely to cause a large-scale epidemic than influenza B

The main cause of this flu is influenza A (H3N2) subtype virus, which belongs to influenza A (hereinafter referred to as influenza A) virus, which is highly contagious and spreads quickly. "Many large-scale influenza outbreaks in history are related to it," said Guo Wei.

The influenza virus not only spreads quickly, but also causes severe symptoms in infected people. In addition to the common symptoms of upper respiratory tract infection (runny nose, sore throat, cough), infected people may also experience a high fever of over 39 degrees Celsius, accompanied by chills, headaches, muscle pains and other discomforts.

In addition, the influenza virus family does not only include influenza A virus, there are also influenza B virus (hereinafter referred to as influenza B) and influenza C virus (hereinafter referred to as influenza C), but the latter two are not as famous as the former.

"In fact, influenza A and B viruses are similar in morphology, structure and biological function, but statistical results show that the probability of influenza A is three times that of influenza B. The reason why influenza A can cause large-scale transmission is mainly related to its great 'changeability'." Chen Chunqi explained that influenza virus is an RNA (ribonucleic acid) virus. According to the different combinations of hemagglutinin antigen (HA) and neuraminidase antigen (NA) on the surface of virus particles, influenza A virus can be further divided into various subtypes, theoretically up to 198 types. More importantly, influenza A virus lacks the participation of RNA proofreading enzymes in the replication process, so it is easy to make mistakes, and the characteristics of segmented genomes make it easy for different subtypes of influenza A virus to reassort genes when infecting a host at the same time.

"During the replication of the influenza A virus, point mutations occur in the HA gene and NA gene, leading to antigenic drift, which can re-infect previously immune hosts and cause repeated epidemics of influenza. This is one of the important reasons why we have to get seasonal influenza vaccines every year." Chen Chunqi introduced that due to the high frequency of mutations in the influenza A virus during the replication process, the probability of viral genome reassortment and even the production of new viruses is also high, but scientists still find it difficult to accurately predict the exact time when a new influenza A virus will appear.

In contrast, the mutation rate of influenza B virus is relatively slow, only 10% to 20% of that of influenza A virus, and it rarely undergoes major mutations. Many people who have had influenza B virus will have antibody immunity, so influenza B virus usually only spreads locally and in a small area, and rarely causes large-scale spread.

"The symptoms of influenza B are relatively mild. Infected people usually experience gastrointestinal symptoms such as nausea, vomiting, and diarrhea, which are similar to what people often call gastrointestinal colds. The influenza C virus only causes subtle or mild upper respiratory tract infections and rarely causes epidemics," said Chen Chunqi.

Vaccination is the most effective means of prevention

Today, in order to fight the COVID-19 epidemic, strict prevention and control measures have been implemented throughout the country, effectively controlling the spread of the COVID-19 virus among the population. However, facing the aggressive influenza, some people may have such questions: influenza and COVID-19 are both respiratory infectious diseases, so why can we control the former but not the latter?

"This is more due to the different levels of attention we pay to influenza and COVID-19. The Law of the People's Republic of China on the Prevention and Control of Infectious Diseases stipulates that my country divides infectious diseases into Class A, Class B and Class C. The higher the level of the infectious disease, the stricter the management measures taken against it. Influenza is a Class C infectious disease, while COVID-19 is a Class B infectious disease, but now they are managed as Class A infectious diseases," Guo Wei analyzed.

Even if someone is confirmed to be infected with the flu virus, the public does not need to panic or be anxious. "A few patients need to be hospitalized for complications, but most infected people recover within a week." Guo Wei said that after being infected with the virus, it is important to avoid cross-infection among family members. It is best to isolate the infected person, such as living in a separate room and not having close contact with other family members.

Regarding prevention, Guo Wei said that currently, influenza vaccination is the most effective means of prevention, which can significantly reduce the risk of influenza and serious complications. The influenza vaccines currently approved for marketing in my country include trivalent inactivated vaccine (mainly for people aged 6 months and above), trivalent live attenuated vaccine (mainly for people aged 3 to 17 years), and quadrivalent inactivated vaccine (mainly for people aged 3 years and above). The trivalent influenza vaccine mainly prevents H1N1, H3N2 and BV influenza; in addition to preventing the above three types of influenza, the quadrivalent influenza vaccine can also prevent BY influenza.

Washing hands frequently is one of the simplest and most important measures to prevent the spread of influenza. "Wash hands promptly after touching public items or sneezing. In addition, ventilate frequently and keep the home environment clean. During the peak season of influenza, people susceptible to influenza should try to avoid crowded and closed places as much as possible," Guo Wei added.

Further reading: "Flu magic medicine" is a prescription drug and cannot be purchased and taken on your own

With the high incidence of influenza in the south during summer, netizens in many places have recently said that the antiviral drug oseltamivir, known as the "miracle flu drug", is out of stock, and in some places it is even difficult to find the drug.

In this regard, Guo Wei, deputy chief physician of the respiratory department of Tianjin Children's Hospital, introduced that oseltamivir is a neuraminidase inhibitor, which inhibits the neuraminidase activity of influenza A and B viruses, thereby inhibiting the release of viruses from infected cells and reducing the further spread of influenza A and B viruses in the human body. It can relieve influenza symptoms and shorten the course of influenza.

Clinically, oseltamivir can be used to treat influenza A and B in adults and children aged 1 year and over, and can also be used to prevent influenza A and B in adults and adolescents aged 13 years and over. "However, oseltamivir is a prescription drug and can only be purchased with a doctor's prescription. It cannot be purchased and taken on your own," said Guo Wei.

In fact, oseltamivir is not the only anti-influenza virus drug available. According to the Diagnosis and Treatment Program for Influenza (2020 Edition), there are three types of anti-influenza virus drugs currently on the market in China: neuraminidase inhibitors, hemagglutinin inhibitors, and M2 ion channel blockers. Neuraminidase inhibitors include not only oseltamivir, but also zanamivir and peramivir.

Guo Wei emphasized that from a medical point of view, taking oseltamivir does have the effect of preventing influenza, but this is not a conventional prevention method. Doctors usually only recommend medication for prevention for people who have close contact with influenza patients and are prone to severe illness from influenza. The best way to prevent influenza is to get a flu vaccine.

Source: Science and Technology Daily

Written by: Science and Technology Daily reporter Chen Xi