This autumn and winter flu season, should we be more careful about influenza or new crown?

At the beginning of the outbreak of the new crown epidemic, the virus was very dangerous, often causing high fever, pneumonia and other symptoms, and the fatality rate was much higher than that of influenza. On 30 January 2020, the World Health Organization declared the pandemic a "Public Health Emergency of International Concern", and countries have introduced emergency responses.

As the alpha, delta, and Omicron variants replace the former one by one and become the most common strains, the symptoms of new crown infection are slowly decreasing. Coupled with the increasing number of people infected with the virus and vaccinated, our lives are slowly returning to normal.

3 years and 9 months after the discovery of the new coronavirus, how serious are the variants circulating today? Is it really not much different from the flu? This autumn and winter flu season, should we be more careful about influenza or new crown?

Influenza and coronavirus, similar in structure but different

Both influenza viruses and coronaviruses are RNA viruses that have a wrap that envelops their RNA material. When infecting a cell, the envelope can fuse directly with the cell membrane to transport the genome into the cell.

Both influenza and coronavirus have spike protein. This is a class of glycoproteins that decorate the viral envelope and are responsible for pairing with target cells. After pairing, the virus's RNA can enter the cell, and the virus parasitizes on the host cell, using the latter's "tools" to replicate more virus.

The two viruses have different spike proteins and target different cells. The new coronavirus spike protein binds to the ACE2 protein, the main function of this protein is to regulate blood pressure and salt balance, and there are cells containing ACE2 protein in many organs and tissues such as lungs, heart, kidneys, intestines, brains, and fat.

The target of the spike protein of the influenza virus is a sugar chain, sialic acid. Although sialic acid is present on most cell surfaces, the spike protein of influenza viruses finds respiratory epithelial cells and preferentially binds.

The spike protein structure of influenza (left) and the new coronavirus (right) is different, and the receptors targeted are also different丨USA Today

Depending on the structure, the type and duration of symptoms are different

Biology often says: structure determines function. Because the cells that are preferentially paired are not the same, the symptoms of influenza are more often concentrated in the respiratory tract, while new crown infection is more likely to trigger symptoms in other parts.

Loss of smell and taste is a common symptom of coronavirus infection, possibly because the spike protein of the new coronavirus infects olfactory epithelial cells in the nasal cavity, causing these cells to fail. And the sense of smell affects the sense of taste, so some people have the new crown and eat nothing tasteless.

Some people infected with the new crown also experience symptoms such as diarrhea and muscle aches. The Fujian Provincial Hospital team counted the symptoms of hundreds of new crown patients infected with the Omicron strain and the 2009 H1N1 flu and found that the proportion of new crown patients feeling muscle aches was three times that of flu patients.

The team also compared the frequency of flu-like symptoms in the two groups of patients and found that the frequency of fever, nasal congestion, sore throat, cough, and headache in influenza patients was twice or more than that of new crown patients.

However, it should be noted that when the H1N1 epidemic in mainland China in 2009, universal testing was not promoted, so only patients with severe symptoms may go to the hospital. The hospital performed a flu test, recorded symptoms, and later included the data in the study; The new crown is a regular nucleic acid test for all residents, so mild and even asymptomatic infections will be screened out, resulting in mild symptoms on average.

National nucleic acid testing丨 cq.gov.cn / Photo by Xinhua News Agency reporter Li Ran

Even so, it took an average of 4 days for influenza patients to turn negative for nucleic acid, while it took 17 days for new crown patients. All flu patients were given the standard drug oseltamivir (trade name Tamiflu), but according to previous studies, oseltamivir shortened the duration of symptoms by about 1 day. There are still 12 days left, how to explain?

The researchers believe that the patient may have been infected with other types of influenza, and the antibodies in the body, although not exactly matched, can also identify the pathogen. Specific immunity in the body works to produce relevant immune cells that clear the virus faster.

From the perspective of mortality, American researchers calculated the risk of death of hospitalized patients with new crown and influenza in the fall and winter of 2022~2023, and the result was that in high-risk groups (the average age of participants was 73 years old), the mortality rate of new crown was still higher than that of influenza.

There were 8,996 hospitalizations due to the new crown, and 538 died within 30 days, with a mortality rate of 5.98%.

There were 2,403 hospitalizations due to influenza, and 76 died within 30 days, with a mortality rate of 3.16%.

Since the fall of 2022, Omicron has become the main strain circulating in the United States, and many Americans have been infected with early new coronavirus variants, which is similar to our current situation.

Figure 丨 Picture worm creativity

If Omicron is reinfected, will the symptoms be milder?

So, having already "yang" once, will the symptoms of reinfection be milder?

At the end of 2022, the first peak of infection, the BA.5 strain of Omicron was mainly circulating in mainland China; and now it is more common for XBB and EG.5.1 strains.

The study found that people infected with BA.2 had 87% immunity to BA.4 or BA.5 within 3-6 months, and a 74% chance of being free from infection within 7-8 months. For the XBB strain, immunity within 3-6 months is only 74%, and 7-8 months after infection, immunity drops to 49%; It has since dropped to 37 percent. Moreover, this Singapore study only counted patients who came to the hospital and were nucleic acid-positive, and did not include asymptomatic carriers and people who did not go to the hospital.

EG.5 is also a variant developed from XBB, according to the news released by the World Health Organization on August 9 this year, it is believed that EG.5 may have growth advantages and immune escape characteristics, which may lead to an increase in cases and hospitalizations, and may become a mainstream strain, but no change in disease severity has been found, and the public health risk of other strains is similar.

Since December 2019, the mutation process of the new coronavirus has nextstrain.org/ncov

Another study took serum from people infected with BA.5, added different variants of the new coronavirus, and measured how many antibodies could be produced. It was found that infection with the XBB strain produced more than 7 times fewer antibodies than reinfection with BA.5.

However, these two articles mainly discuss the possibility of reinfection, and do not discuss whether the symptoms after infection are severe. That is to say, infection with the previous year's strain BA.5 can produce some immunity, but this year may still be re-infected with XBB or EG.5, and the symptoms will not necessarily be milder than last year.

In any case, do a good job of protection

The mainland recently announced that targeted vaccination for key groups will recommend vaccination containing XBB variant antigen components. The target population is the elderly aged 60 and above and the high-risk groups aged 18-59 with more serious underlying diseases and immunocompromised.

These key groups should also try to get the flu vaccine before the peak of influenza. The Chinese Center for Disease Control and Prevention has just released the "Technical Guidelines for Influenza Vaccination in China (2023-2024)", which recommends that all people aged 6 months ≥ without contraindications to vaccination receive influenza vaccine, especially the elderly aged 60 and above, people with chronic diseases, children aged 6 to 59 months and their family members and other key and high-risk groups.

Figure 丨 Picture worm creativity

Even if you are not in a key group, it is recommended that you take protective measures, wash your hands carefully, keep your distance, ventilate well, wear a mask in crowded areas, and minimize contact with others if you have symptoms. After all, these actions can protect you and your loved ones at the same time.

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Author: Liu Siqi

Editor: You Zhiyou

Cover image source: Figureworm Creative