The impact of interventions and modes of contact on the transmission of SARS-CoV-2

A recent study examines the impact of various interventions, the resulting contact patterns and the transmissibility of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), in the context of the evolving epidemiological situation in Italy .

The current study, which is published on the preprint server medRxiv*, found that contact was reduced under tight restrictions, with increased relative contact among household members. This study provides an important quantification of the expected contribution of different levels of restriction in forming social contacts, which can be used to reduce the potential for transmission of SARS-CoV-2.

To study: Investigate the relationship between SARS-CoV-2 interventions, contact patterns and transmissibility. Image Credit: Kzenon / Shutterstock.com

Background

The 2019 coronavirus disease (COVID-19) pandemic, which is caused by infection with SARS-CoV-2, increased rapidly in Italy in the fall of 2020. Shortly after this outbreak, the Italian government introduced a three-level restriction system as part of non-pharmaceutical interventions to increase physical distance and reduce contact.

To this end, the Ministry of Health assigned a yellow, orange or red level to each of the 21 Italian regions and autonomous provinces (AP), in which each color of the three levels corresponded to increasing levels of restrictions. The determination of a region’s risk level was completed after an epidemiological risk assessment based on a combination of quantitative indicators such as the estimated level of transmission and the burden on the health system.

However, it is not well quantified to what extent the different measures promoting physical distancing modify the mixing patterns and the speed with which the population adapts their social interactions to the continuous changes in restrictions.

“A fundamental question that remains open is whether the change in mixing patterns associated with the restrictions implemented can be used to anticipate the effect of a given intervention on the transmissibility of SARS-CoV-2. “

To address this, the current writers investigated changes in the number of contacts and mixing patterns related to the gradual tightening of non-pharmaceutical interventions adopted by the Italian government during the second wave of the COVID-19 pandemic.

About the study

In the present study, researchers conducted a contact survey in the metropolitan city of Milan, located in the Lombardy region of Italy, between July 2020 and March 2021 through online interviews. In addition, the data was collected in collaboration with the Centro Medico Sant’Agostino (CSM), a private health center that is part of the National Health Service.

The researchers interviewed a total of 1,785 people. Of these, 40 study participants (2.2%) reported testing positive for SARS-CoV-2 immunoglobulin G (IgG) antibodies prior to the interview.

In the survey, researchers collected information on the number and type of daily contacts made by study participants during the pandemic when interventions of different intensity were in place. The researchers presented a detailed description of the sample and the average number of daily contacts per person, recorded under different levels.

From this data, the researchers assessed the impact of the restrictions adopted on human mixing models and, through the use of mathematical modeling, estimated their effect on the net reproduction number of SARS-CoV-2. .

The researchers defined a contact as a physical interaction or two-way conversation of at least five words in the physical presence of another person and for each contact reported.

Study results

Regardless of age, the researchers found that the introduction of the tiered restriction system strongly influenced the number of social interactions of study participants.

The researchers compared their observations to the contacts observed before the introduction of the tiered restriction system and found that this corresponded to a relative reduction in the total number of contacts of 18.9%, 34.6% and 45.9%. for light, moderate and heavy restrictions. , respectively.

The researchers reported various observations on the mixed models. To this end, the most marked changes were observed among young people under 20 years of age. However, people aged 20 to 29 were the least affected by the level shift. While inactive people showed a higher reduction in contact compared to students and employed people, participants from larger households reported a higher number of daily contacts across the board.

Serologic test results from participants interviewed below the white level indicated that participants who tested negative for SARS-CoV-2 IgG antibodies prior to the interview reported a slightly higher average number of contacts.

Examining age-specific mixing patterns under different levels, the researchers noted that in addition to reducing the overall number of social interactions, higher restriction levels also increased the relative contribution of young adult contact. (aged 30 to 50) with individuals of the same age. and with people under 10 years old. This observation is probably due to the interactions that occur between household partners, as well as between parents and their children.

At the end of the summer, the number of daily contacts per participant increased significantly. This increase can be explained by a temporal relaxation of social distancing, outside the mandate of personal protective equipment (PPE), given an improved epidemiological situation compared to spring 2020. However, the results also highlight that the Christmas holidays could temporarily have increased contact occurring indoors with non-family members.

A) The colored rectangles represent the levels assigned to Lombardy during the study period. The blue line represents the average number of daily contacts over time with a respective 95% CI (area shaded in blue). B) Bars represent the average number of contacts reported by study participants in different months, stratified by indoor and outdoor contact. Gray lines represent 95% confidence intervals of the total number of daily contacts per person over different months. VS) The dotted and solid lines represent, respectively, the proportions of contact occurring indoors and contact occurring with homeless members over different months.

In addition, the researchers measured the impact of restrictions on the transmissibility of SARS-CoV-2. To this end, the reproduction number of SARS-CoV-2 has been estimated to decrease by 18.7%, 33.4% and 50.2% below the yellow, orange and red levels, respectively. Despite an analysis of contact-only models, in which there was no direct knowledge of actual transmission patterns, these estimates of the relative reduction in transmissibility potential are consistent with previous results.

Conclusion

The present study provided estimates of changes in the number of contacts and mixing patterns associated with the gradual tightening of non-pharmaceutical interventions and their potential impact on the transmission of SARS-CoV-2.

The researchers found an overall gradual temporal decrease in the average number of contacts recorded by study participants, from 16.4% under light restrictions (yellow level) to 45.6% under heavy restrictions (red level). . The breeding number of SARS-CoV-2 has also been estimated to decrease with increasing restrictions.

This study confirms the strong relationship between the observed dynamics and the changes characterizing the levels attributed to Lombardy during the study period. Thus, these results can help anticipate the impact of imposed interventions.

*Important Notice

medRxiv publishes preliminary scientific reports that are not peer reviewed and, therefore, should not be considered conclusive, guide clinical practice / health-related behavior, or treated as established information.