Social contact patterns during the early COVID-19 pandemic in Norway: insights from a panel study, April to September 2020 (preprint)

BackgroundDuring the COVID-19 pandemic, many countries adopted social distance measures and lockdowns of varying strictness. Social contact patterns are essential in driving the spread of respiratory infections, and country-specific measurements are needed. This study aimed to gain insights into changes in social contacts and behaviour during the early pandemic phase in Norway. MethodsWe conducted an online survey among a nationally representative sample of Norwegian adults, including six data collections/waves between April and September 2020, and used survey data from 2017 as baseline. We calculated mean daily contacts, and estimated age-stratified contact matrices that were used to estimate reproduction numbers during the study period. ResultsThe mean daily number of contacts varied between 3.2 (95% CI 3.0-3.4) to 3.9 (95% CI 3.6-4.2) across waves, representing a 67-73% decline compared to pre-pandemic levels. Fewer contacts in the community setting largely drove the reduction; the drop was most prominent among younger adults. Despite gradual easing of social distance measures during the survey period, population contact matrices remained relatively stable and displayed more inter-age group mixing than at baseline. Contacts within households and the community outside schools and workplaces contributed most to social encounters. ConclusionSocial contacts experienced a significant decline during the months following the March 2020 lockdown in Norway, aligning with the implementation of stringent social distancing measures. The findings contribute valuable empirical information into the social behaviour of the Norwegian population during the early pandemic, which can be used to enhance policy-relevant models for addressing future crises when mitigation measures might be implemented.

A standardised protocol for assessment of relative SARS-CoV-2 variant severity, with application to severity risk for COVID-19 cases infected with Omicron BA.1 compared to Delta variants in six European countries (preprint)

Several SARS-CoV-2 variants that evolved during the COVID-19 pandemic have appeared to differ in severity, based on analyses of single-country datasets. With decreased SARS-CoV-2 testing and sequencing, international collaborative studies will become increasingly important for timely assessment of the severity of newly emerged variants. The Joint WHO Regional Office for Europe and ECDC Infection Severity Working Group was formed to produce and pilot a standardised study protocol to estimate relative variant case-severity in settings with individual-level SARS-CoV-2 testing and COVID-19 outcome data during periods when two variants were co-circulating. To assess feasibility, the study protocol and its associated statistical analysis code was applied by local investigators in Denmark, England, Luxembourg, Norway, Portugal and Scotland to assess the case-severity of Omicron BA.1 relative to Delta cases. After pooling estimates using meta-analysis methods (random effects estimates), the risk of hospital admission (adjusted hazard ratio [aHR]=0.41, 95% CI 0.31-0.54), ICU admission (aHR=0.12, 95% CI 0.05-0.27), and death (aHR=0.31, 95% CI 0.28-0.35) was lower for Omicron BA.1 compared to Delta cases. The aHRs varied by age group and vaccination status. In conclusion, this study has demonstrated the feasibility of conducting variant severity analyses in a multinational collaborative framework. The results add further evidence for the reduced severity of the Omicron BA.1 variant.

Severe outcomes in unvaccinated COVID-19 cases <18 years during different variant waves in Norway (preprint)

Objectives We used linked individual-level data from national registries to compare the risk of severe outcomes among unvaccinated COVID-19 cases <18 years between waves of the SARS-CoV-2 Alpha, Delta and Omicron variants in Norway. Methods Our outcomes were hospitalisation with acute COVID-19 or multisystem inflammatory syndrome in children (MIS-C). We calculated adjusted risk ratios (aRR) with 95% confidence intervals (CIs) using multivariable log-binomial regression, adjusting for variant wave, demographic characteristics and underlying comorbidities. Results We included 10,538 Alpha (21 hospitalised with acute COVID-19, 7 MIS-C), 42,362 Delta (28 acute COVID-19, 14 MIS-C) and 82,907 Omicron wave cases (48 acute COVID-19, 7 MIS-C). The risk of hospitalisation with acute COVID-19 in cases <1 year was lower in the Delta (aRR: 0.28, 95% CI: 0.16-0.89) and Omicron wave (aRR: 0.41, 95% CI: 0.20-0.81), compared to the Alpha wave. We found no difference in the risk for this outcome for Omicron compared to Delta in any age group. The risk of MIS-C was lower in the Omicron wave compared to the Alpha (aRR: 0.09, 95% CI: 0.03-0.27) and Delta wave (aRR: 0.26, 95% CI: 0.10-0.63). Conclusions We found no evidence of a difference in the risk of hospitalisation due to acute COVID-19 among unvaccinated cases <18 years for Omicron compared to Delta, but a reduced risk among cases <1 year in Omicron and Delta waves, compared to Alpha. Results also suggest a decrease in the risk of MIS-C in the Omicron wave compared to the Alpha and Delta waves.

Age and product dependent vaccine effectiveness against SARS-CoV-2 infection and hospitalisation among adults in Norway: a national cohort study, July - November 2021. (preprint)

Background: COVID-19 vaccines have been crucial in the pandemic response and understanding changes in vaccines effectiveness is essential to guide vaccine policies. Though the Delta variant is no longer dominant, understanding vaccines effectiveness properties will provide essential knowledge to comprehend the development of the pandemic and estimate potential changes over time. Methods: In this population-based cohort study, we estimated vaccine effectiveness against SARS-CoV-2 infections, hospitalisations, intensive care admissions, and death using Cox proportional hazard models, across different vaccine product regimens and age groups, between 15 July and 31 November 2021 (Delta variant period). Vaccine status is included as a time-varying covariate and all models were adjusted for age, sex, comorbidities, county of residence, country of birth, and living conditions. Data from the entire adult Norwegian population were collated from the National Preparedness Register for COVID-19 (Beredt C19). Results: The overall adjusted vaccine effectiveness against infection decreased from 81.3% (confidence interval (CI): 80.7 to 81.9) in the first two to nine weeks after receiving a second dose to 8.6% (CI:4.0 to 13.1) after more than 33 weeks, compared to 98.6% (CI: 97.5 to 99.2) and 66.6% (CI: 57.9 to 73.6) against hospitalisation respectively. After the third dose (booster), the effectiveness was 75.9% (CI: 73.4 to 78.1) against infection and 95.0% (CI: 92.6 to 96.6) against hospitalisation. Spikevax or a combination of mRNA products provided the highest protection, but the vaccine effectiveness decreased with time since vaccination for all vaccine regimens. Conclusions: Even though the vaccine effectiveness against infection wanes over time, all vaccine regimens remained effective against hospitalisation after the second vaccine dose. For all vaccine regimens, a booster facilitated recovery of effectiveness. The results from this support the use of heterologous schedules, increasing flexibility in vaccination policy.

Vaccine effectiveness with BNT162b2 (Comirnaty, Pfizer-BioNTech) vaccine against reported SARS-CoV-2 Delta and Omicron infection among adolescents, Norway, August 2021 to January 2022 (preprint)

Background: COVID-19 vaccination was recommended for adolescents in Norway since August 2021. In this population-based cohort study, we estimated the BNT162b2 vaccine effectiveness against any PCR-confirmed (symptomatic or not) SARS-CoV-2 infections caused by the Delta and Omicron variant among adolescents (12-17-years-old) in Norway from August 2021 to January 2022. Methods: Using Cox proportional hazard models, we estimated the BNT162b2 vaccine effectiveness against any Delta and Omicron infections. Vaccine status was included as a time-varying covariate and models were adjusted for age, sex, comorbidities, county of residence, country of birth, and living conditions. Data were obtained from the National Preparedness registry for COVID-19, which contains individual-level data from national health and administrative registries. Findings: Vaccine effectiveness against Delta infection peaked at 68% (95%CI: 64-71%) and 62% (95%CI: 57-66%) in days 21-48 after the first dose among 12-15-year-olds and 16-17-year-olds respectively. Among 16-17-year-olds that received two doses, vaccine effectiveness peaked at 93% (95%CI: 90-95%) in days 35-62 and declined to 84% (95%CI: 76-89%) in 63 days or more after the second dose. For both age-groups, we found no protection against Omicron infection after receiving one dose. Among 16-17-year-olds, vaccine effectiveness against Omicron infection peaked at 53% (95%CI: 43-62%) in 7-34 days after the second dose and decreased to 23% (95%CI: 3-40%) in 63 days or more after vaccination. Vaccine effectiveness decreased with time since vaccination for both variants, but waning was observed to occur faster for Omicron. Interpretation: Our results suggest reduced protection from BNT162b2 vaccination against any SARS-CoV-2 infection caused by the Omicron variant compared to the Delta. In addition, waning immunity was observed to occur faster for Omicron. The impact of vaccination among adolescents on reducing infection and thus transmission is limited during omicron dominance. Funding: No funding was received.

Milder disease trajectory among COVID-19 patients hospitalised with the SARS-CoV-2 Omicron variant compared with the Delta variant in Norway (preprint)

Using individual-level national registry data, we conducted a cohort study to estimate differences in the length of hospital stay, and risk of admission to an intensive care unit and in-hospital death among patients infected with the SARS-CoV-2 Omicron variant, compared to patients infected with Delta variant in Norway. We included 409 (38%) patients infected with Omicron and 666 (62%) infected with Delta who were hospitalised with COVID-19 as the main cause of hospitalisation between 6 December 2021 and 6 February 2022. Omicron patients had a 48% lower risk of intensive care admission (aHR: 0.52, 95%CI: 0.34–0.80) and a 56% lower risk of in-hospital death (aHR: 0.44, 95%CI: 0.24–0.79) compared to Delta patients. Omicron patients had a shorter length of stay (with or without ICU stay) compared to Delta patients in the age groups from 18–79 years and those who had at least completed their primary vaccination. This supports growing evidence of reduced disease severity among hospitalised Omicron patients compared with Delta patients.

Age and product dependent vaccine effectiveness against SARS-CoV-2 infection and hospitalisation among adults in Norway: a national cohort study, January to September 2021. (preprint)

Background: SARS-CoV-2 vaccines show high effectiveness against infection and (severe) disease. However, few studies estimate population level vaccine effectiveness against multiple COVID-19 outcomes, by age and including homologous and heterologous vaccine regimens. Methods: Using Cox proportional hazard models on data from 4 293 544 individuals (99% of Norwegian adults), we estimated overall, age-, and product-specific vaccine effectiveness against SARS-CoV-2 infection, hospitalisation, ICU admission and death in Norway, using data from national registries. Vaccine status was included as time-dependent variable and we adjusted for sex, pre-existing medical conditions, country of birth, county of residence, and crowded living conditions. Results: Adjusted vaccine effectiveness among fully vaccinated is 72.1% (71.2-73.0) against SARS-CoV-2 infection, 92.9% (91.2-94.2) against hospitalisation, 95.5% (92.6-97.2) against ICU admission, and 88.0% (82.5-91.8) against death. Among partially vaccinated, the effectiveness is 24.3% (22.3-26-2) against infection and 82.7% (77.7-86.6) against hospitalisation. Vaccine effectiveness against infection is 84.7% (83.1-86.1) for heterologous mRNA vaccine regimens, 78.3% (76.8-79.7) for Spikevax (Moderna; mRNA-1273), 69.7% (68.6-70.8) for Comirnaty (Pfizer/BioNTech; BNT162b2), and 60.7% (57.5-63.6) for Vaxzevria (AstraZeneca; ChAdOx nCoV-19; AZD1222) with a mRNA dose among fully vaccinated. Conclusion: We demonstrate good protection against SARS-CoV-2 infection and severe disease in fully vaccinated, including heterologous vaccine regimens, which could facilitate rapid immunization. Partially vaccinated were less likely to get severe disease than unvaccinated, though protection against infection was not as high, which could be essential in making vaccine prioritisation policies especially when availability is limited.

No difference in risk of hospitalisation between reported cases of the SARS-CoV-2 Delta variant and Alpha variant in Norway (preprint)

ObjectivesTo estimate the risk of hospitalisation among reported cases of the Delta-variant of SARS-CoV-2 compared to the Alpha variant in Norway. We also estimated the risk of hospitalisation by vaccination status. MethodsWe conducted a cohort study on laboratory-confirmed cases of SARS-CoV-2 in Norway, diagnosed between 3 May and 15 August 2021. We calculated adjusted risk ratios (aRR) with 95% confidence intervals (CIs) using multivariable binomial regression, accounting for variant, vaccination status, demographic characteristics, week of sampling and underlying comorbidities. ResultsWe included 7,977 cases of Delta and 12,078 cases of Alpha. Overall, 347 (1.7%) cases were hospitalised. The aRR of hospitalisation for Delta compared to Alpha was 0.97 (95%CI 0.76-1.23). Partially vaccinated cases had a 72% reduced risk of hospitalisation (95%CI 59%-82%), and fully vaccinated cases had a 76% reduced risk (95%CI 61%-85%), compared to unvaccinated cases. ConclusionsWe found no difference in the risk of hospitalisation for Delta cases compared to Alpha cases in Norway. Further research from a wide variety of settings is needed to better understand the association between the Delta variant and severe disease. Our results support the notion that partially and fully vaccinated persons are highly protected against hospitalisation with COVID-19. HighlightsO_LIThe SARS-CoV-2 Delta variant has dominated in Norway since July 2021 C_LIO_LIThere was no difference in the risk of hospitalisation for Delta cases compared to Alpha C_LIO_LIPartially and fully vaccinated cases had >70% decreased risk of hospitalisation C_LI

Trajectories of hospitalisation for patients infected with SARS-CoV-2 variant B.1.1.7 in Norway, December 2020 – April 2021 (preprint)

ABSTRACT Background The SARS-CoV-2 variant of concern (VOC) B.1.1.7 has spread worldwide and has been associated with increased risk of severe disease. Studies on patient trajectories and outcomes among hospitalised patients infected with B.1.1.7 are essential for hospital capacity planning. Methods Using linked individual-level data from national registries, we conducted a cohort study on cases of SARS-CoV-2 in Norway hospitalised between 21 December 2020 and 25 April 2021. We calculated adjusted hazard ratios using survival analysis to examine the association between B.1.1.7 and time from symptom onset to hospitalisation, and length of stay (LoS) in hospital and an intensive care unit compared to non-VOC. We calculated adjusted odds ratios using logistic regression to examine the association between B.1.1.7 and mortality (up to 30 days post discharge) compared to non-VOC. Results We included 946 B.1.1.7 patients and 157 non-VOC. The crude median time from symptom onset to hospitalisation was 8 days (IQR: 5–10) for B.1.1.7 and 8 days (IQR: 4–11) for non-VOC. The crude median LoS in hospital was 5.0 days (IQR: 2.6–10.0) for B.1.1.7 patients and 5.1 days (IQR: 2.5–9.9) for non-VOC. Fifty-four (6%) B.1.1.7 patients died, compared to 14 (9%) non-VOC. There was no difference in the unadjusted or adjusted estimates of our outcome measures for B.1.1.7 and non-VOC patients. Conclusions B.1.1.7 does not appear to influence hospitalised patient trajectories, compared to non-VOC. These findings, along with the success of ongoing vaccination programmes, are encouraging for ongoing capacity planning in the hospital sector.

COVID-19 among immigrants in Norway, notified infections, related hospitalizations and associated mortality: A register-based study.

Aim: Research concerning COVID-19 among immigrants is limited. We present epidemiological data for all notified cases of COVID-19 among the 17 largest immigrant groups in Norway, and related hospitalizations and mortality. Methods: We used data on all notified COVID-19 cases in Norway up to 18 October 2020, and associated hospitalizations and mortality, from the emergency preparedness register (including Norwegian Surveillance System for Communicable Diseases) set up by The Norwegian Institute of Public Health to handle the pandemic. We report numbers and rates per 100,000 people for notified COVID-19 cases, and related hospitalizations and mortality in the 17 largest immigrant groups in Norway, crude and with age adjustment. Results: The notification, hospitalization and mortality rates per 100,000 were 251, 21 and five, respectively, for non-immigrants; 567, 62 and four among immigrants; 408, 27 and two, respectively, for immigrants from Europe, North-America and Oceania; and 773, 106 and six, respectively for immigrants from Africa, Asia and South America. The notification rate was highest among immigrants from Somalia (2057), Pakistan (1868) and Iraq (1616). Differences between immigrants and non-immigrants increased when adjusting for age, especially for mortality. Immigrants had a high number of hospitalizations relative to notified cases compared to non-immigrants. Although the overall COVID-19 notification rate was higher in Oslo than outside of Oslo, the notification rate among immigrants compared to non-immigrants was not higher in Oslo than outside. Conclusions: We observed a higher COVID-19 notification rate in immigrants compared to non-immigrants and much higher hospitalization rate, with major differences between different immigrant groups. Somali-, Pakistani- and Iraqi-born immigrants had especially high rates.