COVID-19 is a Public Health Emergency of International Concern (18) for which the Sri Lankan health sector proactively and timely implemented action, based on the already existing National Influenza Pandemic Preparedness Plan (19). The country mobilized all relevant stakeholders under the readiness of responding to emergencies under the International Health Regulations core capacity development (20). The assistance from the security forces was received to complement the work by the health sector in curtailing the COVID-19 outbreak. Prior to the emergence of the Navy cluster, there were several other clusters of COVID-19 patients which were successfully managed in Sri Lanka. However, the Navy cluster was quite unique. Firstly, it marked the occurrence of the largest number of cases within a confined population. Secondly, if the COVID-19 cluster within the Navy personnel was not successfully managed, it would have led to the paralysis of the functional capacity of the Sri Lanka Navy. This would have had serious repercussions on border security concerns, as an island nation.
The Epidemiology Unit in collaboration with the Sri Lanka Navy conducted a detailed epidemiological investigation on the outbreak in the Navy cluster experienced in the Sri Lanka Navy Base at Welisara and their associated operational units. However, as per the preliminary investigations, a susceptible population of 10400 Navy personnel were identified and considered for the running of the model. As a part of the operational response, the SIR model was used to predict the probable number of infected persons amongst the Navy Cluster.
Under the (0) social distancing scenario, the daily case load was predicted to peak up to 3658 cases on the 49th day. Further, under the best predictable social distancing scenario (74%), the number of daily cases would peak to 2555 on the 54th day. The network of designated hospitals and preventive health services were ready to anticipate and receive a large influx of COVID-19 confirmed cases from the Sri Lanka Navy based on this case prediction scenario. A series of COVID-19 treatment hospitals with increasing bed capacity was identified as a response activity. However, country was able to control this outbreak situation in a very effective manner, implementing different preventive strategies, in addition to the social distancing, probably accounting to more stringent than even 74%.
It was also found that the observed number of daily cases and observed cumulative number of cases seems to be much higher at the initial stages of the outbreak than the predicted number of daily cases and the predicted cumulative number of cases. This may signal the dangerous levels that the outbreak would have escalated into, if not successful control measures were implemented. However, subsequently, both the projected number of daily cases and the cumulative number of cases became lower than the reported number of daily cases and the cumulative number of reported cases.
More ever, the predicted number of COVID-19 cases within the Navy Cluster based on all four different social distancing scenarios were much higher than the actual number observed. When comparing the proportion of cases prevented in relation to the no social distancing scenario, 2.3%, 7.9% and 21.1% were prevented by 25%, 50% and 74% social distancing scenarios. However, when comparing the proportion of actual number of cases prevented compared to the no social distancing scenario, the prevented percentage rose to 90.3.
One possible reason for the large gap between the predicted and actual number of cases observed could at least be partly attributed to the assumptions on which SIR model is based on. The SIR model assumes a fully connected population, where there is homogeneous mixing of infected and susceptible population (21). If the afore mentioned mixing falls anything less than 100%, the mode will have a bias of overestimating the number of cases that would arise. Hence, during the scenario under study among the Naval personnel, one reason why the model provided overestimates could be that the level of mixing in the actual population to be less than the model assumed to be.
In addition, the observed significant reduction of cases could be attributed to cumulative effect of multiple, timely and rigorous outbreak control measures including social distancing implemented by the Ministry of Health and the Sri Lanka Navy. In order to reduce the congestion within the camps, susceptible population was redistributed to a series of designated centers under strict quarantine procedure. In the meantime, the close contacts including family members of the Navy personnel who returned home from the Naval base were sent for institutional quarantine. Further, testing of all susceptible individuals was carried out for COVID-19. The other forces supported the Sri Lanka Navy to collaboratively work on institutional quarantining together with public health authorities to institute relevant preventive measures on timely basis.
The COVID-19 patients were identified and isolated irrespective of if they were symptomatic or not. Firstly, this indicates that the number would have been lower, if only symptomatic patients were accounted for. This would have actually have contributed to an overestimating of the observed number of COVID-19 patients. Secondly, the similar strict preventive health measures were implemented in a uniform manner irrespective of if the positively diagnosed COVID-19 patients were symptomatic or not, despite contrasting evidence on infectivity of asymptomatic COVID-19 positive patients (22–25). This highlights again the effects of stringent preventive health measures that would have contributed to the outbreak control.
Further, the predicted Ro under different social distancing scenarios during the current study ranged from 3.54, 3.36, 3.07 and 2.83 under zero, 25%, 50% and 74% social distancing scenarios. Such effect by social distancing on the control of the spread of the diseases has been shown elsewhere in the world. For example, a large number of COVID-19 cases have been prevented due to rigorous preventive health measures in the aircraft carrier USS Theodore Roosevelt which arrived in Guam in March 2020, which was a similar semi-confined population (2). In the meantime, the R0 in the Diamond Princess Cruise Ship was to be 4 times higher than to the R0 in Wuhan. As per the modelling, it was found that the initial R0 to be 14.8 which came down to 1.78 with the isolation and quarantine measures (3).
When considering the observed daily reported case trend of the Sri Lanka Navy Cluster, the typical bell-shaped epidemiological curve was not seen. This again could explain the vigorous interventions carried out to curtail the outbreak.