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Greece imposed a nationwide lockdown in March 2020 to mitigate transmission of severe acute respiratory syndrome coronavirus 2 during the first epidemic wave. We conducted a survey on age-specific social contact patterns to assess effects of physical distancing measures and used a susceptible-exposed-infectious-recovered model to simulate the epidemic. Because multiple distancing measures were implemented simultaneously, we assessed their overall effects and the contribution of each measure. Before measures were implemented, the estimated basic reproduction number (R_{0}) was 2.38 (95% CI 2.01–2.80). During lockdown, daily contacts decreased by 86.9% and R_{0} decreased by 81.0% (95% credible interval [CrI] 71.8%–86.0%); each distancing measure decreased R_{0 }by 10%–24%. By April 26, the attack rate in Greece was 0.12% (95% CrI 0.06%–0.26%), one of the lowest in Europe, and the infection fatality ratio was 1.12% (95% CrI 0.55%–2.31%). Multiple social distancing measures contained the first epidemic wave in Greece.

Coronavirus disease (COVID-19), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), emerged in China in December 2019 (

In Greece, the first COVID-19 case was reported on February 26, 2020 (

Daily number of coronavirus disease cases by date of sampling for laboratory testing (25) and timeline of key measures, Greece. Dates of telephone survey are indicated. Asterisks indicate spikes in the number of diagnosed cases at the end of March and late April that correspond to clusters of cases in 3 settings: a ship, a refugee camp, and a clinic. EU, European Union.

Despite an ongoing severe financial crisis and an older population, Greece has been noted as an example of a country with successful response against COVID-19 (

We describe a survey implemented during lockdown in Greece and assess the effects of physical distancing measures on contact behavior. We used these data and mathematical modeling to obtain estimates for the first epidemic wave in the country, during February–April 2020, to assess the effects of all social distancing measures, and to assess the relative contribution of each measure towards the control of COVID-19.

We conducted a phone survey during March 31–April 7, 2020, to estimate the number of social contacts and age mixing of the population on a weekday during the lockdown and on the same day of the week before the pandemic, during mid-January 2020, by using contact diaries (

We estimated the average number of contacts for the prepandemic and lockdown periods. We defined 6 age groups to build age-specific contact matrices, adjusting for the age distribution of the population of Greece, by using socialmixr in R software (R Foundation for Statistical Computing,

To estimate the course of the epidemic, we first estimated the basic reproduction number (R_{0}), the average number of secondary cases 1 case would produce in a completely susceptible population in the absence of control measures. Then, we used social contacts matrices to assess the effects of physical distancing measures on R_{0}. Finally, we simulated the course of the epidemic using a susceptible-exposed-infectious-recovered (SEIR) model.

We estimated R_{0} based on the number of confirmed cases with infection onset dates before the first social distancing measures were adopted, up to March 9, and accounted for imported cases. We used a maximum-likelihood method to obtain the R_{0} and 95% CI, assuming that the serial interval distribution is known (_{0} assuming a shorter serial interval of 4.7 days (

Primary social distancing measures implemented in Greece began on March 11. These measures and the dates implemented were closing all educational establishments on March 11; theatres, courthouses, cinemas, gyms, playgrounds, and nightclubs on March 13; shopping centers, cafes, restaurants, bars, museums, and archaeological sites on March 14; suspending services in churches on March 16; closing all private enterprises, with some exceptions, on March 18; and, finally, restricting all nonessential movement throughout the country on March 23 (

We assessed the effects of these measures on R_{0} through the social contact matrices obtained before and during lockdown, as used in other studies (_{0} is a function of the age-specific number of daily contacts, the probability that a single contact leads to transmission, and the total duration of infectiousness; thus, R_{0} is proportional to the dominant eigenvalue of the social contact matrix (_{0} is equivalent to the reduction in the dominant eigenvalue of the contact matrices obtained for the 2 periods (_{i}_{i}

We estimated the relative reduction in R_{0} in 2 periods: the period of initial measures until the day before lockdown (March 11–22), which included closure of schools, entertainment venues, and shops (reduction δ_{1}); and the period of lockdown (March 23–April 26) (reduction δ_{2}). Because we did not assess social contacts during the period of initial measures, we created a synthetic contact matrix by assuming no school contacts because of school closures, and a reduction in leisure and work contacts (_{1} and δ_{2} (n = 1,000 bootstrap samples).

We used a SEIR model to simulate the outbreak from the beginning of local transmission until April 26, 2020, the day before the originally planned date to ease lockdown measures. Susceptible persons (S) become infected at a rate β and move to the exposed state (E) as infected but not infectious. Exposed persons become infectious at a rate σ, and a proportion _{pre}). I_{pre} cases become symptomatic infectious (I_{symp}) cases at a rate of σ_{s}. We assumed that infectiousness can occur 1.5 days before the onset of symptoms (_{asymp})_{.} We assumed that the infectiousness of subclinical cases relative to symptomatic cases was _{s}, and asymptomatic cases recover (R) at a rate of γ_{asymp} (

Epidemiologic parameters | Value | Comments and references |
---|---|---|

R_{0} (95% CI) | 2.38 (2.01–2.80) | Estimated from data on the number of confirmed cases in Greece by accounting for imported cases and assuming gamma distributed serial interval with mean 6.67 days (SD 4.88 days) (D. Cereda et al., unpub. data, |

Latent period (1/σ) | 3.5 days | Based on an average incubation time of ≈5 days ( |

Percentage ( | 80 | From K. Mizumoto et al. ( |

Symptomatic cases | ||

Length of infectiousness before symptoms, d (1/σ_{s}) | 1.5 | ( |

Duration of infectious period from development of symptoms to recovery, d (1/γ_{s}) | 4.5 | To obtain a serial interval of ≈6 days ( |

True asymptomatic cases | ||

Infectiousness ( | 50 | ( |

Duration of infectious period until recovery (1/γ_{asymp}) | 6 days | The same duration of infectiousness as for symptomatic cases = 1/σ_{s} + 1/γ_{s} |

Modified susceptible-exposed-infectious-recovered (SEIR) model used to estimate the course of the first epidemic wave of coronavirus disease, Greece. Cases are classified into susceptible (S), exposed (E), infectious (I, which is divided into 3 conditions: I_{pre}, before developing symptoms, I_{symp} for clinically ill, or I_{asymp} for true asymptomatic), and recovered (R). We assumed that a proportion (_{s}_{asymp} is the rate of recovery for asymptomatic persons; γ_{s} is the rate of recovery for symptomatic persons.

We derived the transmission rate β from R_{0} and parameters related to the duration of infectiousness (_{0} by drawing values uniformly from the estimated 95% CI (2.01–2.80). We modeled the effect of measures by multiplying β by the parameters δ_{1} and δ_{2}; in which δ_{1} corresponds to the reduction of R_{0} in the period of initial social distancing measures, where δ_{1} was drawn from a normal distribution with a mean of 42.7% (SD 1.7%); and δ_{2} corresponds to the reduction of R_{0} during lockdown, for which δ_{2} was drawn from a normal distribution of 81.0% (SD 1.6%) estimated from the bootstrap on the contact data. To account for the uncertainty in R_{0}, δ_{1}, and δ_{2}, we performed 1,000 simulations of the model and obtained median estimates and 95% CrIs.

We obtained the infection fatality ratio (IFR) and the cumulative proportion of critically ill patients by dividing the reported number of deaths and of critically ill patients (

Because multiple social distancing measures were implemented simultaneously, to delineate the effects of each measure on R_{0}, we used information from the contacts reported on a regular weekday in January 2020 and mimicked the impact of each intervention by excluding or reducing subsets of corresponding social contacts (_{t}) for varying levels of infection control measures (hand hygiene, use of facemasks, and maintaining distance

In total, 602 persons provided contact diaries and reported 12,463 contacts before the pandemic and 1,743 during lockdown (

Covariate | Mid-January 2020 | During lockdown | Reduction of reported contacts, % | ||||
---|---|---|---|---|---|---|---|

Participants, no. (%) | No. (%) | Mean (95% CI) | No. (%) | Mean (95% CI) | |||

Overall | 602 (100.0) | 12,463 (100.0) | 20.7 (18.9–22.5) | 1,743 (100.0) | 2.9 (2.6–3.2) | 86.0* | |

Sex | |||||||

M | 295 (49.0) | 6,218 (49.9) | 21.1 (18.3–23.9) | 934 (53.6) | 3.2 (2.7–3.6) | 85.0 | |

F | 307 (51.0) | 6,245 (50.1) | 20.3 (18.0–22.7) | 809 (46.4) | 2.6 (2.2–3.1) | 87.1 | |

Age, y | |||||||

0–4 | 20 (3.3) | 386 (3.1) | 19.3 (12.8–25.8) | 53 (3.0) | 2.7 (2.2–3.1) | 86.3 | |

5–11 | 58 (9.6) | 2,020 (16.2) | 34.8 (29.1–40.6) | 168 (9.6) | 2.9 (2.6–3.2) | 91.7 | |

12–17 | 83 (13.8) | 2,758 (22.1) | 33.2 (28.4–38.1) | 275 (15.8) | 3.3 (2.3–4.3) | 90.0 | |

18–29 | 74 (12.3) | 1,316 (10.6) | 17.8 (14.4–21.1) | 361 (20.7) | 4.9 (3.1–6.7) | 72.6 | |

30–64 | 209 (34.7) | 4,852 (38.9) | 23.2 (19.5–26.9) | 529 (30.4) | 2.5 (2.2–2.9) | 89.1 | |

| 158 (26.3) | 1,131 (9.1) | 7.2 (5.4–8.9) | 357 (20.5) | 2.3 (1.8–2.7) | 68.4 |

*The reduction in the reported contacts becomes 86.9% after adjusting for the age distribution of the population of Greece.

We noted a change in age-mixing patterns in the contact matrices (

Side-by-side comparisons of age-specific contact matrices in Greece before the coronavirus disease pandemic (January 2020; left) and during lockdown (April 2020; right). A) All contacts; B) contacts at home; C) contacts at work; and D) contacts during leisure activities. Each cell represents the average daily number of reported contacts, stratified by the age group of the participants and their corresponding contacts. In panel A, the diagonal of the contact matrix corresponds to contacts between persons in the same age group, the bottom left corner of the matrix corresponds to contacts between school-age children, and the central part corresponds to contacts mainly in the work environment.

Before lockdown, the estimated R_{0} was 2.38 (95% CI 2.01–2.80). During the first period of social distancing measures, in which schools, entertainment venues, and shops were closed, R_{0} was estimated to decrease by 42.7% (95% CrI 34.9%–51.3%); under lockdown, R_{0} decreased by 81.0% (95% CrI 71.7%–86.1%). Thus, the cumulative measures implemented during lockdown would have reduced R_{0} to <1.0 even if the initial R_{0} had been as high as 5.3 (95% CrI 3.5–7.2). Estimated R_{t} was 1.13 (95% CrI 1.38–1.61) during the period of the initial measures but was 0.46 (95% CrI 0.35–0.57) during lockdown (

The first wave of the coronavirus disease epidemic in Greece (February 15–April 26, 2020), estimated from 1,000 susceptible-exposed-infectious-recovered (SEIR) model simulations. A) Effective reproduction number; B) cumulative number of cases; C) new infections; and D) number of infectious persons by date. Orange lines represent the median estimates, and the light orange shaded areas indicate 95% credible intervals. Gray areas indicate the period of restrictions of all nonessential movement in the country (i.e., lockdown).

We assessed the effect of each measure separately and in combinations (_{0} attributed to each measure was 10.3% (95% CrI 5.2%–20.3%) for the decline in work contacts, 18.5% (95% CrI 10.7%–26.3%) for school closures, and 24.1% (95% CrI 14.8%–34.3%) for the decline in leisure activity contacts. Thus, each measure separately would have reduced R_{0} to <1.0 if the initial R_{0} had been as high as 1.11 for the decline in work contacts, 1.23 for school closures, and 1.32 for the decline in leisure activity contacts. A combination of measures could be effective if the initial R_{0} had been as high as 1.78 for interventions reducing work and school contacts, 1.72 for reducing work and leisure contacts, and 1.43 for reducing school and leisure contacts.

The percentage of decline of R_{0} associated with multiple social distancing measures during coronavirus disease lockdown in Greece and the relative contribution of each measure or combination of measures implemented. Boxplots demonstrate distribution of the estimated percent decline from nonparametric bootstrap on the social contacts data based on 1,000 bootstrap samples. R_{0} reduction during lockdown was obtained by comparing social contacts data collected for April 2020 versus January 2020. The other estimates were derived by using the information from contact diaries in January 2020 corresponding to a regular school or work day and excluding or reducing subsets of social contacts at school, work, home, and leisure activities, based on observations during lockdown. Because contact with a particular person can take place in multiple settings, we assigned contacts at multiple locations to a single location by using the following hierarchical order: home, work, school, leisure activities, transportation, and other locations. Dotted line indicates the minimum reduction needed to bring R_{0} from 2.38 to <1. Box top and bottom lines indicate 25th and 75th percentiles; horizontal lines within boxes indicate medians; whiskers indicate 25th/75th percentile plus 1.5 times the interquartile range. R_{0}, basic reproduction number.

We assessed alternative scenarios with less disruptive social distancing measures. A 50% reduction in school contacts, such as smaller class sizes; 20% in work contacts, such as teleworking for part of the population or rotating weekly schedules in which employees telework some days and work onsite other days; and 20% in leisure activities could reduce R_{0} to <1.0 for initial levels as high as 1.32 (95% CrI 1.27–1.38). An even larger decline in leisure activities (50%) could successfully reduce an initial R_{0} as high as 1.48 (95% CrI 1.35–1.62).

Finally, we assessed the increase in R_{t} when measures were partially lifted after lockdown_{t} would remain <1.0 assuming

Estimated R_{t} after the partial lifting of social distancing measures at the end of the first coronavirus disease epidemic wave in Greece for varying effectiveness levels of infection control measures, such as hand hygiene, use of masks, maintaining social distances, in reducing susceptibility to infection. R_{t} during lockdown was 0.46. For the partial lifting of measures, we hypothesized a scenario in which contacts at work and school contacts will return to 50% lower than pre-epidemic levels and leisure activities will return to 60% lower than pre-epidemic levels. Dotted line indicates the threshold of R_{t} = 1. Boxplots of the distribution of the estimated Rt from nonparametric bootstrap on the social contacts data based on 1,000 bootstrap samples. Box top and bottom lines indicate 25th and 75th percentiles; horizontal lines within boxes indicate medians; whiskers indicate 25th/75th percentile plus 1.5 times the interquartile range. R_{t}, effective reproduction number.

By April 26, 2020, Greece had 2,517 diagnosed COVID-19 cases, 23.0% of which were imported, and 134 deaths (

On the basis of the number of deaths and critically ill patients reported in Greece by April 26, and using the number of infections obtained from the model as denominator, we estimated the IFR to be 1.12% (95% CrI 0.55%–2.31%) and the cumulative proportion of critically ill patients to be 1.55% (95% CrI 0.75%–3.22%). As a validation, we estimated the number of deaths by applying a published age-adjusted estimated IFR to the number of infections predicted by the model (

Greece and other countries managed to successfully slow the first wave of the SARS-CoV-2 epidemic early in 2020. Assessing the burden of infection and death in the population and quantifying the effects of social distancing was necessary because the stringent measures taken had major economic costs and restricted individual freedom. In addition, several countries, including Greece, began seeing COVID-19 cases increase after resuming economic activities and travel, indicating the need to reimplement some types of location-specific physical distancing measures.

We assessed the effects of social distancing by using a social contacts survey to directly measure participants’ contact patterns during lockdown in a sample including children. To our knowledge, only 2 other diary-based social contacts surveys have been implemented during COVID-19 lockdown, 1 in China (

We estimated that R_{0} declined by 81% and reached 0.46 during lockdown. This finding agrees with findings from a study pooling information from 11 countries in Europe, which also reported an 81% reduction in R_{0} (

We further attempted to delineate the effects of each measure. For example, many countries, including Greece, instituted large-scale or national school closures (_{0} of ≈1.1–1.3 to <1.0. Only multiple social distancing measures would be effective for reducing an R_{0} at the initial level (2.38) observed in Greece. The finding concerning an 18.5% reduction in R_{0} related to school closures agrees with recent studies suggesting that this measure likely is much less effective for COVID-19 than for influenza-like infections (

By May 18, 2020, Greece had one of the lowest reported COVID-19 death rates in Europe, 15.2 deaths/1 million population (

The first limitation of our study was that, due to the absence of prepandemic data on social contacts, we asked respondents to report their contacts ≈2 months prior to the survey to ensure reports were not affected by increased awareness of the pandemic. Recall bias might be observed, although to what direction is not clear. A general limitation in contact diaries is that participants record a fraction of their contacts (_{0} depends on the serial interval. Because no data from a local study of infector–infectee pairs were available, the distribution of the serial interval was based on previous estimates (_{0} aligned with estimates obtained in China (

Overall, the social distancing measures Greece put in place in early March 2020 had a substantial impact on contact patterns and reduced R_{0} to <1.0. By the end of April, the spread of COVID-19 was contained in Greece, and the country had one of the lowest ARs in Europe after the first pandemic wave. However, as social distancing and travel restrictions are relaxed, close monitoring of R_{t} is essential in order to adapt interventions over time without having to resort to stringent measures. Measuring social mixing patterns and adherence to infection control measures through repeated surveys can be additional tools for real-time monitoring of the epidemic potential in the months to come.

Additional information and formulas used to calculate effects of social distancing measures during the first epidemic wave of severe acute respiratory syndrome coronavirus 2, Greece.

These senior authors contributed equally to this article.

This article was preprinted at

We thank the personnel of the National Public Health Organization for performing the epidemiological surveillance of SARS-CoV-2 in Greece.

The social contacts survey was funded by the Hellenic Scientific Society for the Study of AIDS and Sexually Transmitted Diseases, Athens.

Dr. Sypsa is an associate professor of epidemiology and medical statistics in the Medical School of the National and Kapodistrian University of Athens in Greece. Her research interests include the epidemiology and mathematical modeling of hepatitis C, HIV, influenza, and other infectious diseases, as well as infectious diseases among prisoners and persons who inject drugs.