On 1 July 2020 the entry of tourists into Greece was permitted through 41 points of entry, with 27 airports, 7 ports and 7 ground crossings open to international arrivals. The completion of a Passenger Locator Form (PLF) was required at minimum 48 h prior to check in/arrival for all tourists arriving by air, sea and land. Within this form, detailed information was collected regarding the travellers’ point of departure, duration of previous stays in other countries and address of their stay in Greece. Targeted testing upon arrival via polymerase chain reaction (PCR) was performed at Greek points of entry (1). The epidemiology of Coronavirus Disease 2019 (COVID-19) in the country of origin or in countries previously visited before travelling to Greece, were among the risk assessment criteria used in a testing algorithm upon arrival. After completion of the PLF a personal quick response (QR) code was sent to the traveller and based on the information contained therein, travellers were classified into one of two categories. Depending on their unique QR code, screening personnel either directed travellers to a screening area or allowed them to enter the country. Travellers tested for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) by a trained health team were recommended to self-isolate at the address of their final destination (as declared in their PLF) until screening results were available, with quarantine ending provided that the test result was negative.

Reopening of borders during the summer of 2020 reflected one stage of lifting restrictive measures implemented in the context of COVID-19. A national lockdown beginning on 23 March 2020 was imposed in an effort to reduce the spread of SARS-CoV-2, by which point 695 COVID-19 cases had been recorded in Greece with only five in Crete; four COVID-19 cases had been recorded in the regional unit (RU) of Heraklion and one case in the RU of Lasithi. Beginning 4 May 2020, the national lockdown was gradually lifted with a requirement for face masks to be worn in areas of intense crowding (including means of public transport, supermarkets, hospitals etc.). A total of 2,632 cases were recorded up to the start of lifting restrictive measures, and all RUs of Crete (Heraklion, Lasithi, Chania and Rethimno) presented a low COVID-19 incidence with 0–4 cases per 100,000 population. On 18 May 2020, mobility between RUs within the country was permitted and in late May 2020, eating establishments were reopened. Between mid-May to June 2020, schools gradually reopened with senior classes the first to commence. From 15 June 2020 the two busiest Greek airports located in Athens and Thessaloniki reopened, accepting flights from specific countries; starting 1 July 2020 the entry of tourists into Greece via any airport from all countries was permitted.

Crete has two national airports located in Heraklion and Chania, as well as a municipal/local airport located in Lasithi RU. Ports are located in each of Crete’s four RUs. From January to June 2020 (which includes the period of national lockdown), a total of 161,736 and 35,304 arrivals were recorded by air to Heraklion and Chania, respectively. After reopening the points of entry in Greece, a total of 198,868 arrivals by air were recorded in Heraklion and 46,693 in Chania in July, followed by: 249,104 and 61,101 in August; 183,556 and 41,727 in September; 103,619 and 16,127 in October; 5,393 and 829 in November for Heraklion and Chania, respectively. Moreover, during the summer period nearly 1,200,000 tourists arrived on ferries through Crete’s ports.

The aim of this study was to consider if the presence of a high level of tourism and interaction of tourists is sufficient to affect the long term course of COVID-19 in the destination region. We assessed the seropositivity of the population of Crete—a tourist-based island—compared to the entire population of Greece, in order to investigate if there is any possible effect of the tourist wave on the long term course of COVID-19 pandemic.

The study was initially designed as a national cross-sectional survey—including Crete—and repeated at monthly intervals (2–4). We used the leftover sampling methodology in order to collect serum samples (residual sera) from the general population throughout Greece. As we described in already published scientific articles, a geographically stratified sampling plan based on regional units [Nomenclature of Territorial Units for Statistics (NUTS) level 3] was applied in order to produce a representative sample, taking into consideration age group (0–29, 30–49, 50–69, and ≥70 years) and sex (2–4). The required sample size was determined to be 380 serum samples from each of the 13 NUTS level 2 regions and the sample size for each regional unit (NUTS level 3) from the corresponding region was calculated according to population distribution. As a result, 380 serum samples corresponded to Crete. However, the actual number of collected samples differed from the pre-determined number of samples above. As described in more detail below, we adjusted our results according to the regional unit’s population and their characteristics, sex and age.

The leftover serum samples were collected from a nationwide laboratory framework, including both private microbiological laboratories as well as microbiological and biochemical laboratories of public hospitals in Greece. During the study period, six of the participating laboratories were located in Crete. The samples were derived from individuals who visited the laboratories for routine screening and reasons unrelated to COVID-19. All the confirmed COVID-19 cases were excluded from the sampling. Here, we present the results for the region of Crete, compared to the entire country from June to December 2020, as Crete is a popular tourist-based island that attracts a high influx of tourists.

We have already published findings regarding SARS-CoV-2 seroprevalence in Greece during the pre-vaccination pandemic era, from March to December 2020 (2–4). According to our results, a higher seroprevalence was calculated in December 2020 among younger age groups of “0–29” and “30–49”, as well as in highly populated metropolitan areas (4).

All the participants’ blood samples were collected in a sterile tube. Whole blood was allowed to clot and then centrifuged to separate the serum. The serum was carefully removed with a fine-bore pipette to avoid extracting red cells, and transferred aseptically to a sterile vial labelled with the patient’s identifier, date of collection and specimen type. The leftover serum samples were frozen at −20°C or lower and transported to our laboratory on frozen ice packs appropriately and as soon as possible. They were placed sealable in plastic bags containing absorbent materials. Styrofoam boxes were used to contain the sealed bags. The frozen ice packs were placed at the bottom and along the sides of the styrofoam box. The samples were placed in the centre and more ice packs were placed on top. After receiving of the samples, we tried to test them as soon as possible. In case this was not possible, they were stored in −80°C.

The same method of samples’ preservation and shipment was followed either in the case of private or public laboratories. All the required materials for samples storage and transport were sent from our laboratory in order to the appropriate conditions to be ensured.

As we mentioned in previous articles, the presence of anti-SARS-CoV-2 IgG antibodies was determined using the AB-BOTT SARS-CoV-2 IgG assay, a chemiluminescent microparticle immunoassay (CMIA), with the ARCHITECT i2000SR analyzer (Abbott, Illinois, United States) (2–4). Anti-spike IgG antibodies are used as a marker of prior SARS-CoV-2 infection. The method was validated in our laboratory. We used 305 pre-COVID-19 samples (obtained in 2017) as negative controls and 94 samples from patients with positive SARS-CoV-2 PCR and different symptom durations. The kit displayed 84.0% sensitivity [95% confidence interval (CI): 76.6–91.5] and 99.7% specificity (95% CI: 98.2–100). Given that vaccines were not available during the study period (June–December 2020), all positive samples for IgG anti-SARS-CoV-2 were provoked by natural infection.

The statistical analysis applied is identical to the analysis applied for samples between March to April 2020 (2) and May to August 2020 (3).

The samples were anonymized leftover serum samples. Each sample had a unique code and the required data—sex, age, residence and date of blood sampling—were recorded. Health staff from the participating laboratories explained that in the context of a cross-sectional serosurvey, there was possibility of antibodies testing against SARS-CoV-2 and requested consent of the participants. The research protocol was approved by the ethical committee of the Faculty of Medicine, University of Thessaly, Greece (No. 2116).

Of the total amount of collected samples (55,938 samples), 3,785 samples originated from Crete for the seven-month period between June and December 2020 and 2,170 of them (57.33%) were obtained from females. For each sample, age, sex, and date of blood sampling were recorded.

Of the 3,785 collected serum samples, 39 (1.03%) were found positive for anti-SARS-CoV-2 IgG antibodies. According to the monthly distribution of samples, S1 seroprevalence for anti-SARS-CoV-2 IgG antibodies was as follows: 0.23% in June, 0.53% in July, 0.32% in August, 0.72% in September, 0.96% in October, 1.89% in November and 2.41% in December 2020 (Supplementary Table S1: Anti-SARS-CoV-2 IgG antibody seroprevalence, Crete, June 2020, Supplementary Table S2: Anti-SARS-CoV-2 IgG antibody seroprevalence, Crete, July 2020, Supplementary Table S3: Anti-SARS-CoV-2 IgG antibody seroprevalence, Crete, August 2020, Supplementary Table S4: Anti-SARS-CoV-2 IgG antibody seroprevalence, Crete, September 2020, Supplementary Table S5: Anti-SARS-CoV-2 IgG antibody seroprevalence, Crete, October 2020, Supplementary Table S6: Anti-SARS-CoV-2 IgG antibody seroprevalence, Crete, November 2020, Supplementary Table S7: Anti-SARS-CoV-2 IgG antibody seroprevalence, Crete, December 2020). The adjusted results for age, sex, population (S2) and additionally, for accuracy of the laboratory test (S3) are presented in Supplementary Tables S1–S7. After the addition of NPHO data, S3 seroprevalence was modified and S4 was calculated as 0% in June, 0.09% in July, 0.03% in August, 1.22% in September, 1.41% in October, 1.45% in November and 2.58% in December 2020 (Supplementary Tables S1–S7). A percentage <1% for S4 is observed throughout the summer months, while from September onwards a continuous increase is calculated.

Throughout the study period, the most affected age group varied from month to month. For the first three months between June to August, imperceptible differences existed between age groups. However, from September onwards the difference between age groups increased. In September, the “50–69” presented as the most affected age group with S4 = 4.87% while the next two months the two youngest age groups presented higher seroprevalence (October, “0–29”: S4 = 2.63%, “30–49”: S4 = 1,97%, November: “0–29”: S4 = 1.24%, “30–49”: S4 = 3.32%). During December, the seroprevalence in each age group ranged from S4 = 2.13% to S4 = 3.62%, with the exception of the oldest age group whose seroprevalence was calculated as S4 = 0.94%. To better estimate which age group was more affected, we calculated the average seroprevalence for this seven-month period per age group (Figure 1), which proves that the “30–49” age group was the most affected (S4 = 1.34%).

Although males generally presented higher seroprevalence during the study period, no statistically significant difference was observed among the two sexes, apart from the month of October; the S4 was 2.41% in males and 0.42% in females (difference = 4.11%, p = 0.015) (Supplementary Table S5).

Using our data derived from the serosurvey conducted for the whole of Greece, we designed the following diagram which presents the S4 in Greece (excluding Crete) compared to the S4 in Crete during the study period (Figure 2). A trend of increasing seroprevalence is observed beginning in September for both the mainland of Greece and Crete; however, the seropositivity is 4.16 times higher in Greece than in Crete (10.75% vs. 2.58%).

The monthly CFR and IFR are presented in Supplementary Tables S1–S7. Both values remained low and <1% for each of the first four months (June–September). From October onwards, a continuous increase in CFR is observed, which varying between 1.29% and 7.77%. However, IFR remains low and ranges between 0.00% and 0.10%. In Table 1, the monthly S4, CFR and IFR of COVID-19 for both Crete and mainland Greece are summarized for the study period, based on the S4 seroprevalence. Information regarding the CFR and IFR for the mainland of Greece was derived from the previously published article about SARS-CoV-2 serosurveillance in Greece during the pre-vaccination pandemic era (2–4).

However, there are several studies supporting the opposing view, that travellers not only affect the running incidence temporally, but also have long-term epidemiologic impacts inducing higher SARS-CoV-2 transmission rates, determining even further the COVID-19 pandemic course in the region/country of destination (21, 22). Consequently, tourism destination attractiveness is presented as a possible indicator of areas that would be most affected by the pandemic. Certainly, further investigation must be conducted in this field considering many factors, including whether public health travel-related measures are implemented, epidemiological status of the destination region and the SARS-CoV-2 variants that are circulating or introduced.