Tier 3 interventions needed to contain SARS-CoV-2 transmission, especially with emergence of variants

By Susha Cheriyedath, M.Sc.Mar 1 2021

Studies have shown that non-pharmaceutical interventions (NPIs) such as social distancing and partial lockdowns effectively contain severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) transmission. However, because of these measures' huge economic and social cost, the UK introduced a less socially disruptive tier system. This system was aimed at providing a consistent set of control measures with geographic flexibility.

The tiers comprised multiple NPIs determined by lower-tier local authorities (LTLAs) based on their local transmission intensity. Tiers were inconsistent across LTLAs. For example, some areas had under tier 1 left gyms and fitness centers, while others did not. Although this tier system continued until the second national lockdown in November 2020, before the emergence of the new B.1.1.7 variant, little evidence of the effectiveness of this tier system exists.

Assessing the impact of the Tier system on viral transmission between the first and second national lockdowns in the UK

A recent UK-based study measured the impact of the tier system on the pandemic between the first and second national lockdowns in the UK, prior to the emergence of the B.1.1.7 variant. The modeling study combined real-time reproduction number, R_t estimates obtained from UK case, death, and serological survey data with publicly available regional NPI data. The study is published on the preprint server medRxiv*.

The researchers applied these parameters to a Bayesian hierarchical model with latent factors to account for the impact of Tiers on broader national trends and subnational effects. Primary and secondary outcome measures of the study were a reduction in R_t.

The study results showed that at the national level, SARS-CoV-2 transmission increased between July and late September, despite regional differences. Immediately before the introduction of the tier system, R_t averaged 1.3 (0.9 – 1.6) across LTLAs and declined to an average of 1.1 (0.86 – 1.42) 2 weeks later. The decrease in transmission was not just due to the tier system. The researchers found that the impact of Tier 1 was negligible, while Tiers 2 and 3 reduced transmission by 6% (5%-7%) and 23% (21%-25%), respectively.

Number of LTLAs applying interventions at introduction of Tier 1 (green), Tier 2 (yellow), or Tier 3 (red).

This news article was a review of a preliminary scientific report that had not undergone peer-review at the time of publication. Since its initial publication, the scientific report has now been peer reviewed and accepted for publication in a Scientific Journal. Links to the preliminary and peer-reviewed reports are available in the Sources section at the bottom of this article. View Sources

Findings show that interventions at least as strict as Tier 3 are needed to contain viral transmission

The above approach aimed to analyze broader secular trends at the national level along with the tiers at the LTLA level. The model reproduced R_t estimates and broadly captured their changes over time and between LTLAs. The results suggest that 93% of LTLAs would have started to suppress their epidemics tier 3 was implemented in every LTLA by the second national lockdown, but only 29% moved to tier 3.

The relatively small effect sizes in these findings show that interventions at least as strict as tier 3 are needed to contain the transmission, especially considering the new, more infectious variants, at least until a good proportion of the population has been vaccinated and has immunity to the virus.

"We find that tier 1 has almost no effect on transmission beyond the secular 283 national trend, and that Tier 2 yielded only minor reductions in transmission."

According to the authors, this is the first study that assesses the UK tier system's effects for control of SARS-CoV-2 transmission at the regional and national levels. The model used in the study is primarily data-driven and makes minimal assumptions. Since the analysis focused on the period before the emergence of the B.1.1.7 variant and before the rollout of COVID-19 vaccination, it does not consider the potential confounding factors of vaccination. Hence it is not possible to have falsely attributed the impact of vaccination to the impact of Tiers.

The authors state that although mass vaccination will help decrease the severity of interventions over time, the emergence of more infectious variants will make tiers 1 and 2 less effective in controlling SARS-CoV-2 transmission.

"Non-pharmaceutical interventions will remain necessary to control SARS-CoV-2 transmission, particularly in light of newer more transmissible variants, and at least until an efficacious and effective vaccine becomes widely available or much greater population immunity has amassed."

This news article was a review of a preliminary scientific report that had not undergone peer-review at the time of publication. Since its initial publication, the scientific report has now been peer reviewed and accepted for publication in a Scientific Journal. Links to the preliminary and peer-reviewed reports are available in the Sources section at the bottom of this article. View Sources