Tale of three seeding patterns of SARS-CoV-2 in Saudi Arabia

Human mobility patterns are determinants of disease transmission. This is particularly relevant for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) transmission in Saudi Arabia (population 34 million), which experiences three types of population movements. First is the international and domestic movement of pilgrims in Saudi Arabia. On average each month about 1 million incoming pilgrims from 180 countries merge with about 1 million Saudi-national Sunni pilgrims (75% of the Saudi population) in Saudi Arabia's two holy sites and the nearest entry port of Jeddah.¹ Second is the returning Shiite Saudi-national pilgrims (4·9 million Shiite population in Saudi Arabia) who travel to Iran and Iraq for pilgrimage. Men and women older than 60 years are overrepresented among pilgrims. Third is routine travel to and from Saudi Arabia by Saudis, permanent residents, and non-pilgrim visitors for commerce and tourism, estimated to be about 3 million people each month.

We observed three outbreak seeding patterns in Saudi Arabia in the early phase of the coronavirus disease 2019 (COVID-19) epidemic that match the three types of population movements (figure ). First, transmission related to international pilgrims in holy sites in Mecca and Medina region, largely among the non-Saudi population (49 [94%] of 52 cases). Second, transmission among returning Saudi Shiite pilgrims in the eastern province, affecting mostly Saudi citizens (45 [94%] of 48 cases). Third, general non-specific spread by routine international travel in the political and diplomatic hub of Riyadh province, mostly among the Saudi population (12 [67%] of 18 cases). The remaining provinces of Saudi Arabia did not have COVID-19 cases in the early phase. Of the three seeding patterns, by April 2, 2020, the second had the greatest affect on accelerating domestic transmission in Saudi Arabia, with highest acceleration of cases seen in the eastern region (99 cases per 100 000). Per 100 000 population, Mecca and Riyadh had 17 cases and eight cases, respectively, on April 2.

Figure.

COVID-19 distribution in Saudi Arabia on March 15, 2020, by nationality

COVID-19=coronavirus disease 2019.

Saudi Arabia's ten quarantined student evacuees from China were reported on Feb 16 to have tested negative for SARS-CoV-2.² The country's suspension of pilgrimage visas for international visitors on Feb 27 and for all people by March 4 probably helped delay detection of the first case in the Mecca region until March 10. Saudi Arabia implemented comprehensive pandemic mitigation efforts³ incrementally during March 8–21.

In the early phase of the epidemic, the high proportion of COVID-19 diagnoses in the Riyadh region that were among Saudi nationals (67%) might have resulted from entry restrictions on non-Saudi people and increases in returning Saudi citizens who might have been exposed overseas. The outbreak in the eastern region was reported on March 10 among four returning Saudi pilgrims. Saudi Arabia had not recognised the impact of Qom pilgrimage⁴ on SARS-CoV-2 transmission in eastern Saudi Arabia until then. The country's attempt to identify and quarantine returning Saudi pilgrims proved inefficient initially owing to non-direct routes of travel to Saudi Arabia through Gulf Cooperation Council countries. Saudi Arabia then encouraged returning citizens to voluntarily declare travel to Iran and repatriated stranded citizens using special flights.

Saudi Arabia mitigated international and domestic superspreader transmission of SARS-CoV-2 in its international pilgrim sites with early restrictions of access to its holy sites. Saudi Arabia was unsuccessful in limiting transmission among returning Saudi nationals who participated in an unmitigated superspreader event. The ongoing domestic transmission in the country is largely fueled by returning Saudi-national pilgrims.