Respiratory disease transmission routes

Droplet transmission, contact transmission, and air (aerosol) transmission. More than 90% of infections in daily open scenes come from the first two. Aerosol transmission only carries a high risk of infection in special spaces with airtight and extremely poor ventilation.

Tell me a real case that I came across a while ago when I was helping with influenza investigation at the Centers for Disease Control and Prevention: In March this year, 23 people on the entire floor of an Internet company were infected with influenza A. Tracing the source, the earliest infected person was a department administrative girl. She did not wear a mask during the Monday morning meeting. , and handed over sign-in forms and printed materials to 17 colleagues who attended the meeting. Within three days after the meeting, 12 people who attended the meeting had fevers. Among the remaining 11 employees who did not attend the meeting, only two tested positive, and they were those who sat at the table next to her and shared the coffee machine.

Many people have the impression that "respiratory diseases can be transmitted only by coughing on you", which actually refers to the most common droplet transmission. The large droplets emitted by infected people when they cough, sneeze, talk loudly or even sing karaoke are generally 5-10 microns in diameter. They fly up to two meters away and fall to the ground or surface in a few minutes. People who happen to be within the transmission range may be infected if they inhale it. There is also a controversy that has been discussed in the academic circles for a long time: In the past few years, there was a view that the droplet load emitted by normal speech was too low and did not reach the infection threshold. Until a study by the School of Public Health of the University of Hong Kong in 2023 measured that when people make loud calls without wearing masks, the droplet load within 1 meter is actually similar to two light coughs. This can also explain why in many cases of influenza, the infected person did not cough, but just ate hot pot with others, and the people at the same table were infected.

Contact transmission accounts for a higher proportion than everyone thinks. In the case just now, 8 of the 12 infected people received paper materials from the administrator, which is a very typical indirect contact transmission. Direct contact transmission is well understood, such as direct physical contact such as kissing and hugging an infected person at close range. Indirect contact involves touching surfaces such as tabletops, door handles, mobile phone cases, etc. that are contaminated by the patient's droplets, and then touching one's own mouth, nose, eye conjunctiva and other mucous membranes. The virus enters the body along the mucous membranes. I was chatting with a community pediatrician before. He said that half of the cross-infections in the clinic in autumn and winter are caused by children touching the public toys on the consultation table and turning their heads to rub their noses and eyes. Instead, they are not infected by other children's coughs.

As for aerosol transmission, which you have heard a lot about in recent years, there is still no completely unified definition standard in the academic community. Some scholars classify it as a subcategory of airborne transmission, and some believe that the infection conditions are too different from ordinary airborne transmission and should be classified separately. To put it simply, aerosols are extremely small particles with a diameter of less than 5 microns. They can remain suspended in the air for several hours and even float dozens of meters along ventilation ducts. However, in order to reach an infectious concentration, the premise is that they must be in a confined space with almost no ventilation, such as KTV boxes without windows, elevators full of people, and public bathrooms. There was a classic case in Guangzhou in 2020. Residents going up and downstairs in a certain residential building had never met each other. Because the shared sewer leaked water, virus-carrying aerosols drifted into the home, leading to cluster infections. However, this extreme situation basically does not happen in daily open spaces, and there is no need to panic.

To be honest, I have been doing public health education for almost five years, and I really feel that you don’t have to think too hard about the transmission route. Wearing a mask and washing your hands frequently are more effective than any fancy protection. Fans have always asked me before if I have to wear N95 and spray disinfectant all over my body to be safe when I go out. It is really unnecessary. Think about it, if you are in an open park, even if you pass by an infected person, his droplets will either not reach you, or they will be quickly blown away by the wind. The aerosol is so diluted that it cannot even be detected, and it cannot reach the infection dose at all.

Oh, by the way, there is another pitfall that everyone often steps into: many people wear masks and only cover their mouths, leaving their noses exposed, or rub their eyes directly after touching the outside of the mask. This is actually equivalent to directly opening the door to contact transmission. Don’t be too troublesome. When taking off the mask, take the ear strings on both sides and wash your hands after taking it off. Doing these small details well will be much more effective than hoarding a bunch of special medicines.