Evaluating the exclusion of unvaccinated people in the prevention of SARS-CoV-2 transmission

By Neha MathurDec 14 2021Reviewed by Aimee Molineux

In a recent pre-print study posted to the medRxiv* server, a team of researchers performed a planned analysis of a vaccine mandates and vaccine passports (VMVP) study to describe a method that can evaluate the benefits of excluding unvaccinated people to reduce transmissions using a number needed to exclude (NNE) model.

Lockdowns and restrictions imposed during the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic have immensely affected the global population psychologically and socio-economically.

*Important notice: medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Since mass SARS-CoV-2 vaccination campaigns started globally, VMVP for SARS-CoV-2 were considered a path out of the pandemic. Amid the emergence of SARS-CoV-2's new variants of concern (VOCs), several past research studies have enumerated the benefits of higher vaccination coverage. On the other hand, the benefits of excluding unvaccinated people who pose a significant risk of transmitting SARS-CoV-2, from different settings are barely known. 

The study

In the present study, the NNE was calculated in six distinct types of settings: households, social gatherings, casual close contacts, work/study places, healthcare, and travel/transportation for estimating the transmissibility of the Delta variant.The NNE is analogous to the number needed to treat (NNT) and refers to the number of unvaccinated people who need to be excluded from a particular setting (e.g. healthcare) to prevent one SARS-CoV-2 transmission event from the unvaccinated individuals, where NNT=1/ARR. The ARR is the baseline transmission risk in the population for a type of setting, which depends on the secondary attack rate (SAR) observed in that type of setting and the baseline infection risk in the population. 

The NNEs were calculated for each setting using the current (mid-to-end November 2021) baseline infection risk in many countries to help societies and policymakers around the globe decide about excluding unvaccinated people via VMVP. It is worth noting here that vaccine-induced or natural immunity in a population reduces the baseline transmission risks (i.e., the point-prevalence of infectious SARS-CoV-2 cases) in that population for any given setting, increasing the NNE.

Findings

The study’s findings depend on the accuracy of the assumptions of the NNE model and the baseline infection risks estimates. The most important of all the assumptions was that the higher corrected SARs of the model increased the estimated baseline transmission risks from the Delta variant in the household setting by 26%, which lowered the NNEs. The other assumptions were that the SARs for each type of setting is relatively consistent, the baseline infection risk is relatively stable across individuals in a population, and unvaccinated people have no natural immunity.

The first finding suggested that a minimum of 1,000 unvaccinated individuals need to be excluded to prevent one SARS-CoV-2 transmission event in most types of settings for many countries, notably Canada, Australia, California, China, France, Israel, and others. The other study finding revealed that for almost every country examined, the NNEs of ≥ 250 to 333 were within the NNTs range of acetylsalicylic acid (ASA) in primary prevention of cardiovascular disease (CVD). These findings suggest that ASA is not recommended for the primary prevention of CVD, similarly excluding unvaccinated people is not admissible because the drawbacks of doing so outweigh the benefits.

The most notable implication of the study results is is that excluding unvaccinated people is not justifiable because NNEs are high, and baseline transmission risks are negligible for most setting types and across many countries. In addition, the disadvantages of excluding unvaccinated people via VMVP may outweigh the benefits (NNEs ≥ 250 to 333); thus exclusion may not be an appropriate response to the risk that unvaccinated people pose to others.

Conclusions

The study results suggest that the NNEs and the baseline infection risk could be key indicators of the drawbacks and benefits of VMVP. In the future, an online interactive global database of the NNEs and baseline infection risks for many countries may also be developed, integrating vaccination-induced or natural immunity data to explore the impact of herd immunity on the NNE. If this database could integrate data of regional SARS-CoV-2 vaccination rates and SARS-CoV-2-related hospitalizations/deaths that would further enhance NNEs relevance within other metrics. Furthermore, this database may be updated with the transmissibility data of future dominant VOCs (e.g., Omicron), enabling efficient monitoring of the benefits of exclusion vs. its drawbacks in real-time.

*Important notice: medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.