COVID-19 pandemic in Ireland: Epidemiology, public health restrictions and vaccination uptake

Data sources

The methodology used to extract data from national data sources has been reported in detail previously^6–8. Briefly, using data from Ireland’s COVID-19 Data Hub, we calculated weekly number of COVID-19 cases, number of COVID-19-related deaths, average number of cases in hospital per day and average number of cases in critical care per day from 19 July 2020 to 27 March 2021. Data for hospitalized cases are based on aggregate data from 29 acute hospitals, publicly available via the COVID-19 Data Hub. The case counts from the COVID-19 Data Hub relate to confirmed cases of COVID-19 only (defined as “Detection of SARS-CoV-2 nucleic acid in a clinical specimen”)⁹. Data for critical care cases are based on publicly available aggregate data from the National Office of Clinical Audit ICU Bed Information System. Data obtained for the period 19 July 2020 to 27 March 2021 were combined with the equivalent data utilized in our previous work documenting the first wave, providing a dataset spanning the first 13 months of the pandemic in Ireland.

Outbreak settings data were sourced from the Health Protection Surveillance Centre (HPSC) weekly epidemiological surveillance reports on COVID-19 outbreaks/clusters. Our analysis of outbreak settings is limited to data from 5 September 2020 onwards as HPSC ‘outbreak setting specific’ reports used for secondary analysis were not available prior to this date. Private households were not reported for the period January to mid-February 2021, owing to mitigation phase of the third wave. The HPSC designated the timeline for the three waves of the pandemic in Ireland as follows: wave 1 – 1 March 2020 to 1 August 2020; wave 2 – 2 August 2020 to 21 November 2020; wave 3 – 22 November 2020 onwards¹⁰.

Aggregate data for ‘close contacts’ of confirmed cases were extracted from the Health Service Executive (HSE) Contact Management Programme (CMP)¹¹, for the period 19 July 2021 to 27 March 2021 inclusive. The HPSC definition of a close contact was based on that of the European Centre for Disease Prevention and Control (ECDC)¹². This was defined as either an individual who: (i) spent >15 minutes within 2 m distance of; (ii) shared an enclosed space for >2 hours with; (iii) a healthcare worker without Personal Protective Equipment who cared for; (iv) shared transport while seated within two seats of; (v) shared a household with - a confirmed case of COVID-19¹³.

Irish governmental publications, literature and news media timelines were used to document the timeline of Public Health restrictions implemented^14–16. The national HPSC interim case definitions, HSE operational reports and news media timelines of reported events were used to report changes in COVID-19 testing strategy and capacity. Data on COVID-19 vaccine uptake in Ireland and other EU countries are publicly available from the ECDC website. EU/EEA Member States reported basic indicators (number of first, second and unspecified doses)¹⁷.

Analyses were performed using Microsoft Excel software. Open office is a free-to-use alternative that can reproduce the same analyses.

Descriptive epidemiology

COVID-19 epidemiological trends observed during implementation and relaxation of Public Health restrictions were expanded to include the second and third waves of the pandemic in Ireland (19 July 2020 -27 March 2021), in addition to the first wave (1 March 2020 to 18 July 2020)⁶. Weekly estimates of total number of cases, deaths, hospitalisations, critical care admissions, median age of cases and mean number of close contacts per case were extracted from the relevant data source. The HPSC reported on outbreaks across 25 possible settings which we re-categorised into ten categories (Appendix 1).

Using ECDC vaccination data, we plotted weekly vaccination uptake against the evolving epidemiological situation (numbers of cases and deaths) from 29 December 2020 (the date the first COVID-19 vaccine was administered in Ireland¹⁸) to 27 March 2021. Data from this source were also extracted for four other EU countries with comparable populations and geopolitical circumstances to Ireland.

European context

The first three waves of the COVID-19 pandemic in Ireland were compared with four EU countries selected in our earlier study. Epidemic curves for Austria, Belgium, Ireland, Portugal and Sweden representing COVID-19 cases and deaths were constructed using ECDC data reported via The European Surveillance System (TESSy) from 1 February 2020 to 27 March 2021¹⁹. Data from all five countries included deaths from confirmed COVID-19, notified from both hospital and community settings. Belgium included both confirmed and probable COVID-19 deaths²⁰. ECDC data on weekly first-dose vaccine uptake as a percentage of total population was plotted over time beginning week 1 of 2021.

Cases, hospitalisations and deaths

Weekly cases of COVID-19 during wave 2 peaked at 7,452 by week ending 24 October 2020; during wave 3 weekly cases peaked at 43,844 by week ending 9 January 2021 (Table 1). The week ending 28 November 2020 recorded the lowest level of cases between waves 2 and 3 at 1,849. Weekly deaths during wave 2 peaked at 45 by week ending 21 November 2020 (final week of wave 2) and during wave 3 weekly deaths peaked at 385 by week ending 6 February 2021. These compare with peak case and death numbers of 5701 and 316 during wave 1, recorded by weeks ending 18 and 25 April 2020 respectively.

Table 1 reports the average daily number of hospitalised cases and daily number of ICU cases, for each epidemiological week during each wave. Hospitalised and ICU cases peaked in mid-April 2020 during wave 1, in late October 2020 during wave 2, and in late January 2021 during wave 3.

Close contacts

Weekly mean number of close contacts per case was lowest at 0.7 in April 2020, rising to 6.6 by end of the first wave. The figure ranged from 4 to 7 during the second wave, and then fell to 3 contacts per case following increased restrictions in mid-October 2020. During the third wave, the mean number of contacts per case peaked at 5.2 on Christmas week, falling to a low of 1.7 by mid-January.

Age-stratified incidence and vaccination uptake

Figure 2 illustrates the COVID-19 incidence rate among those over 65 years exceeded that among those under 65 years during the first wave, but not during the second or third wave. Incidence rates for both groups peaked concurrently during the third wave; the week ending 9 January 2021. The incidence declined more rapidly among those under 65 years, from a peak of over 900 cases/100,000 population to under 200 cases /100,000 population by week ending 30 January 2021; whereas, the incidence for those over 65 years declined more gradually. First dose vaccine uptake initially increased more rapidly among those under 60 years, but was overtaken by those over 60 years by week ending 6 March 2021.

Figure 2. Weekly COVID-19 incidence per 100,000 population, first and second dose vaccine uptake by age group in Ireland from 1 March 2020 to 27 March 2021.

Outbreak settings

Outbreak settings are reported in Figure 4. Private households were by far the leading outbreak setting reported to HPSC, however, these were excluded from Figure 4 due to missing data. Long-term care facilities and workplaces were the most common outbreak settings during wave 2 and wave 3. Education facilities were the next most common setting during wave 2 and from March 2021 onwards in wave 3.

Epidemiology and vaccine uptake across ECDC states

Figure 3 illustrates COVID-19 cases, deaths and first-dose vaccine uptake across five EU states from 1 January 2020 to 27 March 2021. The surge in cases during the second wave in Austria, Belgium, Ireland, Portugal and Sweden began in late October 2020. The magnitude of this second wave far exceeded that of the first wave in all states except Ireland. Austria, Portugal and Sweden experienced more prolonged second waves, spanning week ending 10 October 2020 to week ending 13 February 2021, with a third wave following in quick succession. Belgium and Ireland experienced comparatively shorter second waves.

Figure 3. COVID-19 epidemic curves showing number of COVID-19 cases, deaths, and % first dose vaccine uptake among 5 EU member states (all with population < 12 million) from January 2020 to March 2021 using ECDC TESSy data.

Figure 4. Weekly number of outbreaks by setting notified from September 2020 to end-March 2021 in Ireland.

The surge in cases during the third wave began earlier in Ireland and Portugal, in week ending 19 December 2020. Its magnitude greatly exceeded that of the second wave, with weekly cases counts five-fold higher in Ireland and two-fold higher in Portugal. The surge in cases during the third wave in Austria, Belgium and Sweden arrived later, beginning week ending 27 February. However, the magnitude of the third wave did not exceed that of the second in these three states.

First-dose COVID-19 vaccination uptake reached 10% by week ending 13 March 2021 in all five EU states. In Ireland and Portugal, the earlier onset of the third wave saw an earlier decline in the weekly case count when vaccination uptake was only 0–5%. In Austria, Belgium and Sweden, the weekly case count of the third wave continued to increase after week ending 13 March 2021.

Main findings of this study

In this paper we describe the second and third waves of COVID-19 pandemic in Ireland which peaked in October 2020 and January 2021 building on our previous study describing the first wave. Our results provide an overview of the evolution of the first 13 months of the pandemic in Ireland and a comparison with the pandemic trajectory in four other European countries. We report three main findings. Firstly, we demonstrate the effectiveness of NPIs and of the co-ordinated public health response in managing outbreaks and preventing disease transmission, subsequent hospitalisations, and deaths. Secondly, the sharp increase in case numbers in the third wave demonstrates how rapidly the disease can spread with easing of restrictions and emergence of novel VOC. Thirdly, we demonstrate how the vaccination programme limited the impact of the third wave, with fewer hospitalisations and deaths than seen in earlier waves of the pandemic.

What this study adds

Similar to findings from other countries and regions, in Ireland the implementation of NPIs assisted outbreak control and limited the impact of the second wave²¹. In contrast with some countries, high levels of compliance with mandatory face mask use in shops and on public transport were reported²². Moreover, the importance of the testing and contact tracing processes in controlling the spread of COVID-19 were widely understood and adhered to by the public in Ireland²³. Despite concerns about non-compliance and ‘pandemic fatigue’, between July and October 2020 support for restrictions and compliance with them increased steadily and appreciably²⁴. In Ireland, regional departments of public health focused on controlling outbreaks in particular settings. During Summer 2020 numerous workplace outbreaks occurred in food and meat processing facilities where large groups of employees work in very close physical proximity in chilled or refrigerated settings with poor ventilation, particularly conducive to airborne transmission of SARS-CoV-2²⁵. In response, a multi-disciplinary, multi-agency National Outbreak Control Team was established to summarise the situation, review the evidence on investigation and control measures, ensure consistency of approach and develop national guidance^26,27.

In Ireland approximately 1 million students attending 4000 facilities were impacted by school closures during Level 5 restrictions and national lockdown, during periods of peak community transmission in Wave 1 and Wave 3^28,29. The return to classroom-based education for primary and secondary school students from September 2020 was supported by a bespoke Public Health medicine response for rapid risk assessment and testing prioritisation of close contacts of cases. While outbreaks associated with educational settings did occur, their relative proportion was much less when compared to other congregate settings, considering the absolute number of education attendees nationally (Figure 4)³⁰. Transmission risk within school settings was mitigated by implementation of infection prevention and control measures including ventilation, physical distancing, respiratory etiquette, hand hygiene, and in the case of staff and secondary school students, adherence to the use of facemasks. Combined with rigorous risk assessment by regional Public Health department teams, this facilitated prevention and control of any suspected outbreak³¹. National guidance during the study period recommended individual assessment for use of face masks in children under 13 years of age, although they were recommended elsewhere in children over 2 years of age^31,32.

The risk of relaxing restrictions in the context of ongoing community transmission is demonstrated by the sharp increase in case numbers in the third wave in Ireland. The government proceeded with a multi-phase reopening of economy and society in the first week of December 2020 despite daily case counts in the 200-300 range, and contrary to advice from the National Public Health Emergency Team (NPHET)^33,34. In mid-December the Alpha VOC (B.1.1.7 variant), which was demonstrably more transmissible than the wild type SARS-CoV-2 virus, was first detected in the UK; it was detected in Ireland soon thereafter, followed by the Beta VOC (B.1.351 variant) by end of December^1–3. This VOC emergence likely contributed to the magnitude of the third wave in Ireland. A national lockdown was instigated on 22 December 2020, however, the Christmas and New Year period saw high levels of inter-generational mixing indoors, which created a ‘perfect storm’ for viral transmission and culminated in an exponential rise in cases in the first two weeks of January 2021^35,36. Overall, the weekly number of deaths during the peak of the first and third wave were similar (316 deaths in late-April 2020 and 385 deaths in early-February 2021), though case numbers were much higher in the third wave. However, the very restricted testing strategy during the first wave may have underestimated true case numbers³⁷.

Vaccination against SARS-CoV-2 became available in late 2020, with first doses administered in Ireland on 29 December 2020. Vaccination policy (as directed by the National Immunisation Advisory Committee) focused initially on groups most at-risk of morbidity and mortality (i.e., long-term care facility residents aged >65 years, and frontline healthcare workers). From our overview of COVID-19 epidemiology in Ireland, there was a marked reduction in the number of cases and outbreaks in long-term care settings from February 2021 once the majority of patients and staff were immunised, thus providing real-world evidence for vaccine effectiveness.

Limitations of this study

Underreporting of cases occurred during the peak of third wave, as the unprecedented surge of cases led to a mitigation phase, and delays in reporting of outbreaks to the national surveillance system. Consequently the number of outbreaks reported at that time is likely an underestimate³⁸. Limited information on source of infection leads to ascertainment bias and potentially biasing reporting of outbreak settings. Private households as the predominant outbreak setting may be due to outbreak ascertainment, i.e., household contacts are more readily identified because residential addresses are routinely collected, and cases are more forthcoming in identifying these contacts compared to those in work or social settings. Nevertheless, the predominance of household settings may be indicative of a true effect of household transmission, as this setting is likely to represent a greater intensity of exposure to cases, facilitating secondary infections. Finally, our analysis of outbreak settings is limited to data from 5 September 2020 onwards. Nonetheless, the case count was relatively low between 19 July 2020 and 5 September 2021 of wave 2 and we reported on the most common outbreak setting, i.e., food processing workplaces^26,27.