Shielding strategy could be ineffective in reducing COVID-19 prevalence and mortality

With the spread of the COVID-19 pandemic, a prime objective of all containment measures has been the need to prevent a sudden upsurge of severe infections with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), limiting the burden on the healthcare services within sustainable boundaries. With the recognition that certain subgroups of the population seemed to have a higher risk of developing severe disease following infection, some countries put shielding policies into place to protect them from infection.

A new study by researchers at the University of Glasgow and the NHS Greater Glasgow and Clyde, and published on the preprint server medRxiv* in September 2020, examines the benefits of such measures.

Shielding of High-Risk Groups

Most Asian countries adopted wide-ranging containment policies, including testing, tracing, and isolating all sections of society alike, based on high test availability and the determination to monitor and ensure the implementation of the strategies devised by the public health authorities. The situation in Europe is different, with population-wide social distancing and hand hygiene being enforced on the one hand, while special risk groups are shielded, or asked to be in strict isolation, for varying periods of time.

This is true even of Sweden, the outstanding exception to the generally uniform implementation of lockdown policies in Europe, which still enforced shielding of such high-risk sections of society. In the UK, two categories of shielded patients are listed, namely, those at high risk and those at moderate risk.

In the UK, shielding of high-risk patients was defined as their not leaving their homes on any but the most urgent pretext, and even avoiding all but essential contact with their household members, while food and other necessities were delivered to them at their homes. The moderate-risk patients, on the other hand, were recommended to follow general population measures carefully.

Issues with Shielding

Some earlier studies indicate that prolonged shielding has adverse effects on the supposed beneficiaries. Another issue is that the high-risk group here is defined on the basis of previous viral outbreaks rather than on a good understanding of SARS-CoV-2. Currently, the evidence is pouring in that the presence of self-reported heart, kidney, or lung disease, as well as diabetes and obesity, are linked to higher odds of serious COVID-19 requiring hospitalization and respiratory support, as well as death in hospital.

Researchers are now working on a COVID-19 risk score to help screen patients for the truly high-risk category who may be advised to shield. An important issue here is the lack of any but mathematical modeling data as to how shielding may contribute to lower adverse outcomes at the population level.

High- vs. Moderate/ Low-Risk Groups

The current paper compares high- and moderate- or low-risk individuals in terms of the infection risk, outcome risk, and the proportional contribution to population-level COVID-19-linked outcomes.  It included a total of ~1,300,000 people, obtained from GP records in a single NHS area in Scotland. Of these, about 2.5% were asked to shield while just over a quarter was at moderate risk.

In the shielding group, over half had severe respiratory disease, about a fifth on dialysis, over 15% on immunosuppressive treatment, while smaller proportions of patients with other medical conditions, including severe heart disease and organ transplants.

The moderate risk category included patients with high blood pressure, comprising a little less than half, while chronic lung disease was present in over 40%. About 40% were 70 or older. A fifth had diabetes, and about 14% of heart disease.

Outcomes in High-Risk vs. Other Groups

The researchers found that of the entire group, just over 1% had undergone COVID-19 testing. The odds of testing increased with age, female sex, and risk category. The prevalence of confirmed COVID-19 in this group was about 0.25% and was again highest in the above categories.

Of over 3,300 confirmed COVID-19 cases, just over 1,600 were hospitalized, and about 120 required ICU admission. The risk of ICU admission was higher in those aged 45-64 than in the oldest age group.

The number of deaths was just over 1,000. Men and women showed equal risk, while it was higher in older people. The risk of mortality increased from the low-risk to the high-risk group, as expected, even after adjustment for sex and socioeconomic deprivation.

The shielded group made up about 9% of confirmed COVID-19 infections, about 15% of hospitalizations, about 5% of all ICU admissions, and 13% of deaths, all related to COVID-19. The risk of hospitalization was 18 times higher in the shielded group, but the ICU admission risk was four times greater relative to the low-risk group.

In the moderate-risk group, the number of infections, hospitalizations, ICU admissions, and deaths made up about 40%, 60%, 20%, and 75% of the total. However, there was no significant difference in mortality between the moderate and high-risk groups.

Those over 70 years or with weakened immunity run a risk of dying equivalent to those in the shielded group. Septuagenarians were 8 times more likely to test positive despite being advised to shield. They were 7 times more likely to die of the infection following testing and had a 74-fold risk of death relative to the low-risk category. They made up 18% of all infections, 23% of hospital cases, and half of all deaths due to COVID-19.


Since three of every four deaths occurred in the moderate-risk category that falls outside shielding recommendations, it is evident that shielding criteria are too narrow to impact mortality rates significantly.

To prevent 80% of total deaths, therefore, shielding would have to cover many more categories, including five that are currently treated as moderate risk, comprising about 30% of the population, rather than the current 2.5% who are currently shielded.

The risk of ICU admission following hospitalization was 14 times less than in individuals above 70 years than for low-risk patients. This is likely to be an effect of having to prioritize care for those with the best prognosis in a situation of overwhelming burden on the healthcare system, rather than being the outcome of effective prevention of infection in this category.

Also, the shielded group continued to test positive at an 8-fold higher rate, to die after confirmed infection at a fivefold higher rate, and to have 49 times the risk of death from COVID-19 overall. The researchers suggest this casts doubt on the effectiveness of the shielding strategy.

Indeed, relative to the low-risk group, those judged to be at moderate risk, who make up a quarter of the population, had four times the odds of testing positive. Mortality after confirmed infection, and overall mortality, was five times and 26 times higher, respectively, in this group. This should serve as a wake-up call to recognize the high risk of many thought to be at moderate risk, particularly those who are older.

Moderate-risk criteria define about a quarter of the population in many studies. However, one US paper reported that 45% of the adult study population had one or more of a long list of coexisting illnesses that are potentially linked to a higher risk of adverse outcomes with COVID-19. were four times as likely to have confirmed infection than the low-risk category.

Shielding of such a large percentage of the population is likely to be both unenforceable and unpopular in view of the increasing trend towards relaxation of lockdown restrictions. Therefore, the researchers conclude, “Shielding is probably best viewed as an individual-level 6 intervention to be used alongside other population-wide interventions such as physical distancing, face coverings, and hand hygiene.”

*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.

Journal reference:
Dr. Liji Thomas

Written by

Dr. Liji Thomas

Dr. Liji Thomas is an OB-GYN, who graduated from the Government Medical College, University of Calicut, Kerala, in 2001. Liji practiced as a full-time consultant in obstetrics/gynecology in a private hospital for a few years following her graduation. She has counseled hundreds of patients facing issues from pregnancy-related problems and infertility, and has been in charge of over 2,000 deliveries, striving always to achieve a normal delivery rather than operative.


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