Third scenario: Contact tracing

52

Figure 6.8. Fraction of cumulative infections averted % relative to the day upon each scenario is implemented. The results show the fraction of cases averted for each scenario A, B, C (compared to the baseline model with ft = 25%) for each day of our period of study. Note that the fraction of infections averted starts when the intervention is implemented, corresponding to 26^th April 2020 for scenario A, 25^th August 2020 for scenario B, and 22^nd December 2020 for scenario C.

Figure 6.9. Effective reproductive number Rt for the different physical distancing interventions (A, B, C) from March 2020 to February 2021. The dotted horizontal line at y = 1 represents the Rt’s threshold value. Recall that scenario A is implemented on 26^th April 2020 and maintained until 25^th August 2020, scenario B is implemented on 25^th August 2020 and maintained until February 2021, and scenario C is implemented from 22^nd December 2020 to February 2021.

53 that are traced and isolated in each scenario correspond to already infected or infectious contacts. But, to estimate the number of contact tracers needed for this effort, we have to count the total number of people that need to be contacted most of whom are not infected.

A. What would happen if the lifting of the first state of emergency on 4^th May 2020 was accompanied by an additional effort in tracing and isolating contacts?

Results for 5% and 10% additional tracing and isolation of infected people implemented from 4^th May 2020 onwards are shown in Figure 6.10.

According to our results, this intervention shows that incrementing 5% and 10% in tracing and isolating infected people from 4^th May 2020 would substantially reduce the burden of morbidity and mortality, as well as the pressure on the healthcare system over the epidemic simulation (Figure 6.10).

In particular, 5% of additional tracing would diminish the impact of the third COVID-19 wave from mid-December 2020 to January 2021. By 10^th February 2021, we would reach a total of 67,048 cases and 1,479 deaths, reducing approximately 94% of infections and deaths compared to the baseline scenario (Figure 6.13a). For this, a total of 5,781 infected people should be traced and isolated, with the peak of tracing occurring on 21^st January 2021, with a daily of 52 newly infected people found (Figure 6.14a). In turn, a workforce of 8-21 contact tracers would be needed to trace these additional infected people and their respective contacts (Table C.4, Appendix C).

Under 10% of additional tracing, the epidemic would end by the beginning of September 2020 (assuming re-infections cannot occur). At the end of the simulation, we would reach a total of 35,846 infections and 924 deaths, averting approximately 98-99% of infections and deaths compared to the baseline model (Figure 6.13a). It would be necessary to trace and isolate a total of 2,413 infected people, with the peak of tracing occurring on 5^th May 2020, with 83 newly infected people traced (Figure 6.14a). In turn, 13-34 contact tracers would be needed to trace the daily number of additional infected people and their respective contacts (Table C.4, Appendix C). From mid-July 2020, only a residual number of contacts would be traced due to the suppression of transmission, with almost no more active cases occurring after that period.

B. What would happen if the school’s reopening on 18^th September 2020 was accompanied by an additional effort in tracing and isolating contacts?

Results for 5% and 10% additional efforts in tracing and isolating infected contacts from schools’ reopening on 18^th September 2020 are shown in Figure 6.11.

According to our results, incrementing 5% and 10% efforts in tracing and isolating infected contacts since 18^th September 2020 would reduce the total number of cases, deaths, and the overwhelming of the

54 healthcare system. In both situations, it would have the potential to diminish the impact of the third epidemic wave from mid-December 2020 to January 2021 (see Figure 6.11), and importantly, to reduce the number of beds occupied in infirmaries and ICUs.

Under 5% of additional tracing, we would attain a total of 307,890 cases and 6,119 deaths by 10^th February 2020, reducing approximately 66-67% of infections and deaths compared to the baseline scenario (Figure 6.13b). For this, a total of 34,507 infected contacts should be traced and isolated, with the peak of tracing occurring on 20^th January 2021, with 399 newly contacts traced (Figure 6.14b). In turn, approximately 60-160 contact tracers would be needed to trace the additional infected contacts and their respective contacts (Table C.4, Appendix C).

Under 10% of additional tracing, we would reach a total of 150,493 infections and 3,233 deaths by the end of the simulation, averting 87-90% of infections and deaths compared to the baseline model (Figure 6.13b).

For this, it would be necessary to trace a total of 21,785 infected people, with the peak of tracing occurring three days after its implementation, on 21^st September 2020, with 245 infected people traced (Figure 6.14b).

To trace these additional infected contacts and their respective contacts, 37-98 contact tracers would be required (Table C.4, Appendix C).

C. What would happen if the second state of emergency, since 17^th November 2020, was accompanied by an additional effort in tracing and isolating contacts?

Results for 5% and 10% additional tracing and isolation implemented on 17^th November 2020, after the second COVID-19 peak wave, are shown in Figure 6.12.

Numerical results show that incrementing 5% and 10% the effort of tracing and isolating infected people from 17^th November 2020 would reduce the burden of morbidity and mortality, as well as the demand on the healthcare system, especially at the third wave from mid-December 2020 to January 2021 (Figure 6.12).

Under 5% of additional tracing, we would reach a total of 573,956 infections and 11,614 deaths by the end of the simulation, averting 31-40% of infections and deaths compared to the baseline model (Figure 6.13c).

These results would be attained if an additional 48,579 infected people were contacted and isolated, with the peak of tracing occurring on 18^th November 2021, with 970 infected people contacted (Figure 6.14c).

We estimated that approximately 146-388 extra contact tracers would be required to trace the additional infected people and their respective contacts (Table C.4, Appendix C).

Under 10% of additional tracing, we would reach a total of 434,141 cases and 9,280 deaths by 10^th February 2020, reducing approximately 66% of infections and 51% deaths compared to the baseline scenario (Figure 6.13c). In turn, it would be needed to trace and isolate a total of 55,442 infected people, with the peak of tracing occurring on 18^th November 2020, with 1,938 people traced (Figure 6.14). We estimated that

55 approximately 291-776 extra contact tracers would be needed to trace the additional infected contacts and their respective contacts (Table C.4, Appendix C). Table C.3, Appendix C, summarises the numerical results obtained for the different 5% and 10% additional contact tracing scenarios presented in this subsection compared to the baseline model.

Figure 6.10. Effect of implementing 5% and 10% additional contact tracing efforts from 4^th May 2020 onwards on the evolution of the number of cases (a,b), hospitalisations (c,d), and deaths (e,f) compared to the baseline model. The horizontal dotted lines in (c,d) represent the basal beds capacity for COVID-19 patients in infirmary and ICU, which we take as a limit of 17,700 and 1,021 beds, respectively. Note that the baseline scenario includes the tracing efforts undertaken by the Portuguese authorities (and corresponds to 0% additional tracing efforts). In all panels, the y-axis is plotted in a logarithmic scale.

56

Figure 6.11. Effect of implementing 5% and 10% additional contact tracing efforts from 18^th September 2020 onwards on the evolution of the number of cases (a,b), hospitalisations (c,d), and deaths (e,f) compared to the baseline model. The horizontal dotted lines in (c,d) represent the basal beds capacity for COVID-19 patients in hospitals and ICU, which we take as a limit of 17,700 and 1,021 beds, respectively. Note that the baseline scenario includes the tracing efforts undertaken by the Portuguese authorities (and corresponds to 0% additional tracing efforts). In all panels, the y-axis is plotted in a logarithmic scale.

57

Figure 6.12. Effect of implementing 5% and 10% additional contact tracing efforts from 17^th November 2020 onwards on the evolution of the number of cases (a,b), hospitalisations (c,d), and deaths (e,f) compared to the baseline model. The horizontal dotted lines in (c,d) represent the basal beds capacity for COVID-19 patients in hospitals and ICU, which we take as 17,700 and 1,021 beds, respectively. Note that the baseline scenario includes the tracing efforts undertaken by the Portuguese authorities (and corresponds to 0% additional tracing efforts). In all panels, the y-axis is plotted in a logarithmic scale.

58

Figure 6.13. Fraction of infections averted under the different contact tracing scenarios relative to the day upon each measure is implemented. The panels show the results for the cumulative infections averted since 4^th May 2020 (a), 18^th September 2020 (b), and 17^th November 2020 (c). All situations (a,b,c) present the projection for 5% and 10% additional tracing efforts. Dashed vertical lines represent the moment of measure’s introduction.

Figure 6.14. Daily additional infected contacts traced under the different contact tracing scenarios from 3^rd March 2020 to 10^th February 2021. The panels show the results of the absolute number of infected people needed to trace per day (without counting for their respective contacts), starting from the moment of its implementation on 4^th May 2020 (a), 18^th September 2020 (b), and 17^th November 2020 (c). All situations (a,b,c) present the projection for 5% and 10% additional tracing efforts. Dashed vertical lines represent the moment of measure’s introduction.

Note that the vertical scales of the three panels (a,b,c) are different.

59