Looking at the impact of the gradual relaxation of social distancing measures on the spread of the new crown from four Chinese cities, Fudan Yu Hongjie’s research team has made important progress

Recently, Hongjie Yu’s research group at Fudan University has made important progress in the field of gradual relaxation of social distancing measures on the influence of people contact patterns and the spread of new coronary pneumonia.
The research results are based on "The impact of relaxing interventions on human contact patterns and SARS-CoV-2 transmission in China "" as the topic, was published online in Science Advances on May 7 in the form of a research article (Research Article).

The study analyzed the changes in the social contact patterns of the population in four major Chinese cities, and revealed the interaction between the contact patterns in homes, schools, workplaces, and communities, and the dynamics of the spread of new coronary pneumonia.

The first author of the study, Zhang Juanjuan, a young researcher at the School of Public Health of Fudan University, introduced the importance of the study for the current epidemic prevention and control.
“Under the background that China has not achieved herd immunity through vaccination and is facing huge pressure from overseas imports, in addition to maintaining nucleic acid testing In addition to specific interventions such as close contact tracking, enhancing personal protection awareness (paying attention to personal hygiene, proactively maintaining social distancing, wearing masks, etc.
) is essential to prevent the rebound of China's new crown pneumonia epidemic.

" ▲Paper published in Science Advances Yu Hongjie The previous study of the group has clarified the impact of non-pharmacological interventions at the individual and group levels on the spread of new coronary pneumonia.
Both articles were published in Science (Zhang J et al, Science 2020, 368 (6498): 1481-1486; Sun K et al, Science 2021, 371 (6526): eabe2424), respectively emphasized the importance of strict measures to increase social distancing taken by China during the outbreak to control the epidemic, and due to the heterogeneity of the spread of the new coronavirus, epidemic control requires case isolation , Coordinated efforts of close contact tracing and group-level intervention measures.

However, the impact of relaxation of intervention measures after the epidemic has eased on the spread of new coronary pneumonia is not yet clear.
The epidemic trend after the unblocking of the epidemic is different in different countries, such as the rebound of the epidemic in many countries.

However, China has not seen a large-scale epidemic rebound in the context of the relaxation of intervention measures (the orderly resumption of work and production from February 10, 2020, and the gradual relaxation of social distancing measures, etc.
) and the continuous import of overseas cases.

Quantitative measurement of the interaction between interventions and population exposure patterns is the key to clarifying the dynamic characteristics of the transmission of new coronary pneumonia after unblocking.

To this end, Hongjie Yu’s research team conducted a telephone survey of social contacts among people in Wuhan, Shanghai, Shenzhen and Changsha from March 1 to May 15, 2020.
9,206 samples were randomly selected and 3 cases of the new crown epidemic were quantitatively measured.
Changes in social contact patterns of all age groups at different stages (before the epidemic, during the blockade, and after unblocking, Figure 1).

 ▲Figure 1.
The number of daily reported cases of new coronary pneumonia (orange-local cases; green-imported cases), investigation time and intervention measures (school closure, home office, traffic control, community management) over time.

A: Wuhan; B: Shanghai; C: Shenzhen; D: Changsha (picture source: research team) The results show that compared with the period of blockade, the average number of contacts per person per day has recovered from 5% to 17% after the blockade was released, but still 3-7 times lower than the level of exposure before the epidemic.

After relaxing the intervention, most of the contact still occurred within the family, and the contact between the workplace and the community (outside the home, school, and workplace) has been restored, but the typical characteristics of peer contact and two-generation contact within the family are still in contact Dominate the matrix (Figure 2).

 ▲Figure 2.
Contact matrix based on age.

AD: The contact matrix of the population in the four cities before the new crown pneumonia epidemic; EH: The contact matrix of the population in the four cities during the lockdown; IL: the contact matrix of the population in the four cities after the unblocking (photo source: research team) The basic reproduction number R0 is An important parameter in the infectious disease transmission model represents the number of second-generation cases caused by one case in a completely susceptible population.

Based on the social contact matrix, population susceptibility, infectiousness and other parameters at different stages above, this study uses the next generation matrix approach to estimate the change in R0 after the intervention measures are relaxed.

In order to clarify the impact of changes in the contact matrix and population susceptibility on the dynamics of disease transmission, the SIR (susceptible, infected, and removed) transmission dynamics model was further constructed.

The results showed that during the lockdown period, the change in exposure patterns caused a significant reduction in R0, and R0 in the four cities was far below the epidemic threshold, indicating that strict non-pharmaceutical interventions are sufficient to interrupt the spread of new coronary pneumonia; The number of daily contacts has increased, but it is not enough to cause R0 to exceed the epidemic threshold, indicating that the degree of recovery of population contacts after unblocking is not sufficient to cause a rebound of the epidemic.

If it is assumed that the susceptibility of the population is the same at all ages, the conclusion remains the same (Figure 3).

▲Figure 3.
The impact of relaxing interventions on disease transmission.

AD: When there are age differences in population susceptibility, the change in R0 after unblocking the four cities; EH: When there is no age difference in population susceptibility, the change in R0 after unblocking the four cities (picture source: research team) Finally, this study Using the above-corrected transmission model, the impact of different intervention measures and relaxation strategies on the emergence of the second wave of epidemics was evaluated.

Taking Shanghai as an example, a scenario analysis was conducted on the degree of exposure and restoration of the population in schools, workplaces and communities.

The results show that if the school is completely closed (no intervention measures such as close contact tracing are taken), if the epidemic is still controllable (R0 remains below the epidemic threshold), the workplace exposure can be restored to up to 55% of the pre-epidemic level.
Community contact can be restored to 65%.

If 80% of community contact is restricted (equivalent to 1.
2 people per day), 50% of workplace contact can be allowed to resume (Figure 4A).

After high school starts, even if only 10% of workplace contacts and 10% of community contacts are restored, R0 can rise above the epidemic threshold (Figure 4B).

The more schools open, the greater the impact on R0 (Figure 4C).

The impact of school closures on the spread of COVID-19 has a non-linear relationship with the degree of recovery of other activities (Figure 4D).

When most work and community activities are restricted, closing the school can reduce R0 by 77%; in contrast, when social activities outside the school return to pre-epidemic levels, closing the school only reduces R0 by 5%.

This shows that only when other social activities are restricted can school contact play a decisive role in the spread of new coronary pneumonia; once other social activities are opened, the effect of closing schools is very limited, because the infection can be spread through other transmission chains.

 ▲Figure 4.
The impact of different recovery levels of school, workplace and community contacts on the spread of the disease.

A: The impact of the degree of restoration of workplace-community contacts on R0 when the school is closed; B: The impact of the degree of restoration of workplace-community contacts on R0 when the high school starts; C: When the school is fully opened, the workplace is in contact with the community The impact of the degree of recovery on R0; D: The impact of different levels of recovery in schools, workplaces and communities on R0 (picture source: research team) This study quantitatively measured the gradual relaxation of intervention measures in Wuhan, Shanghai, Shenzhen and Changsha ( Changes in social contact patterns under the circumstances of orderly resumption of work and production, gradual relaxation of social distancing measures, etc.
) and the impact of such changes on the spread of new coronary pneumonia
The study found that during the gradual relaxation of social distancing measures, the number of people contacted has recovered, but it is much lower than the level before the epidemic and will not cause local transmission of new coronary pneumonia.

The degree of openness of schools and other places has a non-linear relationship with the spread of new coronary pneumonia.
The results of the study once again support the importance of closing schools to control the epidemic during urban lockdowns.

The article also emphasizes that the blockade should be lifted and the normalization of prevention and control phases entered.
On the basis of specific intervention measures such as nucleic acid testing and close contact tracking, the awareness of personal protection should be enhanced (pay attention to personal hygiene, actively maintain social distance, wear masks, etc.
) It is essential to prevent the rebound of the new crown pneumonia epidemic.

Zhang Juanjuan, a young researcher at the School of Public Health of Fudan University, is the first author, a professor at the School of Public Health of Fudan University, Shanghai Institute of Major Infectious Diseases and Biosafety, and the Department of Infectious Diseases of Huashan Hospital Affiliated to Fudan University.
Marco Ajelli is the co-corresponding author.

This research was funded by the Outstanding Youth Fund of the National Natural Science Foundation of China (81525023), the Emergency Research Project of the Shanghai Municipal Science and Technology Commission (20411950100), and the China Postdoctoral Science Foundation (2020T130014ZX).