Danish vs. Swedish information reveals social interplay main threat issue for SARS-CoV-2 an infection
A new study published in September 2020 on the preprint server medRxiv * compares the prevalence of antibodies against the coronavirus 2 (SARS-CoV-2) with severe acute respiratory syndrome in Danish and Swedish health care workers and comes to that Conclusion that “social interaction is one of the greatest risk factors for infection with SARS-CoV-2. “
Since the COVID-19 pandemic began in December 2020, non-pharmaceutical interventions, particularly social distancing, have been in place to limit the spread of the virus. However, health care workers (HCWs) have found it almost impossible to put this into practice. Many previous studies in Denmark and Italy have found that this group has a higher seroprevalence compared to the general population, indicating a higher risk of infection.
What determines the risk in different jobs?
The current study focuses on employees of a rescue organization called Falck with over 30,000 HCWs worldwide. Most employees come into contact with customers or patients and are therefore at the forefront of the pandemic. The question was whether increased social interaction increases the risk of infection.
In the order of interaction with potentially infected customers or patients, ambulance personnel are at the highest risk. In Denmark, ambulance workers are asked to wear masks if they suspect COVID-19. In Sweden, however, they are asked to put on masks in all contact with patients and to use full personal protective equipment (PPE) in the event of COVID. 19 is suspected.
HCWs in clinics have fewer interactions with new cases on a daily basis and may have a lower risk of infection. Office workers are likely to be least at risk. Firefighters also have a part-time job with variable functions and therefore different exposure levels.
The Falck guidelines have changed depending on government measures in Denmark and Sweden. For example, teleconsultations are encouraged where possible, and patients with symptoms suggestive of a potential COVID-19 are asked to stay away from the clinic. All patients are asked to take appropriate hygiene measures to ensure that they do not transmit infections through fomites or personal contact. Disinfection between patients is recommended.
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 over alternating weeks. The aim was to determine the risk for different areas of responsibility and the role of the number of contacts per day that each employee makes at work. The study also examines differences in seroprevalence mediated by regional and national boundaries.
Out of a total of ~ 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 vice versa in Denmark. About half were between 40 and 60 years old.
Around 10% of Danish participants said they came into contact with more than 10 people during their working day, compared to ~ 20% of Swedish workers. Again only ~ 6% of Danish Falck employees are HCWs, but ~ 45% of Swedish employees. Around 26% versus 2% work as fire fighters in Denmark and Sweden respectively.
Almost all of the employees tested had a valid test result, with the seroprevalence after the first and second round of testing being 3.3% and 4.1%, respectively. This increased to 4.9% over the next two rounds after the fourth round.
By country, this corresponds to ~ 3% and ~ 8% of the Danish and Swedish employees. This could be because the two countries handled the pandemic differently. Compared to healthy blood donors in Denmark and Sweden, this is higher with a seroprevalence of 1.7% and 6.8%, respectively, but this is partly due to the natural increase over time.
The lowest seroprevalence was seen in the group aged 60 and over, possibly because this group was more aware of their increased risk and therefore took greater precautionary measures.
Ambulance workers had the highest levels of seropositivity and firefighters the lowest, regardless of country. The number of contacts on a working day was proportional to the percentage of seropositive results in Sweden, but not in Denmark, probably because such a small proportion of employees in the latter country, unlike Sweden, had more than 10 contacts in a single working day.
The risk of seropositivity increases with the number of contacts
By using multiple parameters to adjust the risk such as age, gender, region and type of job, the risk of testing positive for SARS-CoV-2 antibodies increases with employees who come into contact with 11 to 20 contacts a day than doubled relative to those who have no contact with others. For those with more than 20 contacts a day, the risk is nearly tripled.
The mismatched model also showed that ambulance staff were at the highest risk, with more than double that of office staff. HCWs were 30% more at risk, but firefighters were 60% less at risk. However, when the different effects of age, gender, region and contact are taken into account, there was no significant difference in risk between the different groups.
Instead of a function-based risk group, the number of contacts in a working day determines the risk. It is believed that ambulance staff are at particularly high risk as they are required to admit all patients, regardless of whether the latter have symptoms of COVID-19 or not.
Despite the use of masks by ambulance workers in suspected COVID-19 or in all patients in Denmark or Sweden, the seroprevalence in the latter was higher at ~ 15% versus ~ 4%. This can point to an important truth: the masks worn by ambulance workers did not provide protection and it may be advisable to ask patients to wear masks as well.
A similar finding emerged from a recent study of HCW in the Danish capital region, in which paramedics had the highest seroprevalence among hospital staff at almost 5% versus 4%.
The researchers say, “We found that the number of customer or patient interactions during a working day was the most important predictor of seropositivity.” This multi-round, multi-region study is a robust indicator of the true seroprevalence in the group tested. By covering groups with different levels of risk, the study contributes to a more accurate representation of seroprevalence and virus spread. This will help develop better public health policies to protect frontline workers with high levels of social contact, as well as other populations, from infection.
* Important NOTE
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be considered conclusive, guide clinical practice / health-related behavior, or treated as established information.