Danish vs. Swedish data shows social interaction major risk factor for SARS-CoV-2 infection

A new study published on the preprint server medRxiv* in September 2020 compares the prevalence of antibodies to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in Danish and Swedish healthcare workers, to conclude that “social interaction is one of the biggest risk factors for getting infected with SARS-CoV-2.”

From the beginning of the COVID-19 pandemic in December 2020, non-pharmaceutical interventions have been implemented, especially social distancing, to limit the spread of the virus. Healthcare workers (HCWs) have found it nearly impossible to put this into practice, however. Many earlier studies in Denmark and in Italy have found that this group has a higher seroprevalence compared with the general population, indicating their higher risk of infection.

What Determines Risk Level in Different Jobs?

The current study focuses on workers employed by a rescue organization called Falck, with over 30,000 HCWs worldwide. Most employees come into contact with customers or patients and are thus in the frontline of the pandemic. The question was, does increased social interaction boost the risk of infection?

By order of interaction with potentially infected customers or patients, ambulance staff are at the highest risk. In Denmark, ambulance workers are asked to wear masks in case of a patient suspected to have COVID-19, but in Sweden, they are asked to don masks for all contacts with patients and to use full personal protective equipment (PPE) if COVID-19 is suspected.

HCWs at clinics have fewer interactions with new cases every day and might be at a lower risk of infection. The least risk is probably for office workers. Firefighters also have a part-time job with variable functions and, therefore, different levels of exposure.

Falck guidelines have varied with government measures in Denmark and Sweden, respectively. For instance, teleconsultations are being encouraged wherever possible, and patients with symptoms that suggest potential COVID-19 are asked to stay away from the clinic. All patients are asked to ensure they do not transmit infection via fomites or personal contact by proper hygiene measures. Disinfection is recommended between patients.

More Social Contacts Among Swedish Falck Employees

The study tested ~2,000 of the 8,000 Danish employees and ~1,200 of the 2,000 Swedish employees for SARS-CoV-2 antibodies in alternate weeks for two months. The aim was to order the risk level for various job functions, and the role played by the number of contacts per day that each employee makes at work. The study also explores differences in seroprevalence mediated by region and national boundaries.

Of a total ~3,300 participants, 95% were tested at least twice and 64% all four times. The researchers found more women than men in the Swedish arm but the reverse in Denmark. About half were aged 40-60 years.

About 10% of Danish participants said they came into contact with more than 10 people during their workday, vs. ~20% of Swedish workers. Again, only ~6% of Danish Falck employees are HCWs but ~45% of Swedish workers. About 26% vs. 2% work as firefighters in Denmark and Sweden, respectively.

Seropositivity Percentage

Almost all the tested employees had a valid test result, with the seroprevalence being 3.3% and 4.1% after the first and second rounds of testing. This increased over the next two rounds to 4.9% after the fourth round.

By nation, this corresponds to ~3% and ~8% of Danish and Swedish employees. This could be because the two countries dealt with the pandemic differently. As compared to healthy blood donors in Denmark and Sweden, with a seroprevalence of 1.7% and 6.8% respectively, this is higher, but this is partly attributable to the natural increase with time.

The lowest seroprevalence was in the group aged 60 and above, perhaps because this group was more aware of their increased risk and therefore took greater precautions.

Ambulance workers had the highest seropositivity proportion irrespective of countries, and firefighters had the lowest. The number of contacts in a workday was proportional to the percentage of seropositive results in Sweden but not in Denmark, probably because such a small fraction of the employees in the latter country had over 10 contacts in a single workday, unlike in Sweden.

Seropositivity Risk Increases with Number of Contacts

Using multiple parameters to adjust the risk, such as age, sex, region, and type of job, the risk of testing positive for SARS-CoV-2 antibodies is more than doubled in employees who come into contact with 11-20 contacts a day relative to those who have no contact with others. For those with over 20 contacts a day, the risk is nearly tripled.

The unadjusted model also showed that the highest risk was for ambulance staff, at over twice that of office staff. HCWs had a 30% higher risk, but firefighters had 60% reduced risk. However, when the differential effects of age, sex, region, and contacts is included, there was no significant difference in risk between the various groups.

Rather than risk group based on function, the number of contacts over a workday determines the risk. Ambulance staff are thought to be at especially high risk since they are obliged to accept all patients whether or not the latter displays symptoms of COVID-19.

Despite the use of masks by ambulance workers when dealing with suspected COVID-19 or all patients in Denmark and Sweden, respectively, the seroprevalence in the latter was higher, at ~15% vs. ~4%. This may indicate an important truth: the masks worn by the ambulance workers failed to confer protection, and it may be wise to ask patients to wear masks as well.

A similar finding emerged from a recent study on HCWs in Denmark’s Capital Region, where paramedics had the highest seroprevalence among hospital staff, at almost 5% vs. 4%, respectively.


The researchers say, “We found that number of customer or patient interactions during a workday was the most prominent predictor for seropositivity.” This multi-round multi-region study is a robust indicator of actual seroprevalence in the tested group. By covering groups at differing risk levels, the study contributes to a more accurate representation of seroprevalence and virus spread. This will help to build better public health policies to protect frontline workers with a high degree of social contact, as well as other population groups, against infection.

*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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