Prevention of spread of SARS-CoV-2 in Switzerland in summer 2021, in the context of variants of concern and vaccination

Measures to prevent the spread of SARS-CoV-2 work best in combination and when the number of incident cases is low¹. When a set of preventive measures is relaxed, alternative means of infection control are needed to counteract the increase in person-to-person contacts and opportunities for virus transmission.

Most of the tools described here to prevent SARS-CoV-2 transmission have been described before, as part of a strategy to keep case numbers low¹. Vaccination is the newest and single most effective intervention to prevent COVID-19, including that caused by variants of concern. Developments in the national capacity for whole genome sequencing have improved the surveillance and monitoring of variants of concern.

1. Vaccination

Increasing the pace of vaccine roll-out to achieve full vaccination in as high a proportion of all eligible age groups will contribute substantially to reducing the spread of the delta variant. The effectiveness of mRNA vaccines against symptomatic infection caused by the delta variant (87.9%, 95% CI 78.2-93.2%) appears to be only slightly lower than against the alpha variant (93.4%, 95% CI 90.4-95.5%) after two doses¹⁰. Vaccine effectiveness against the delta variant after a single dose was low, however (33.2%, 95% CI 8.3-51.4%).

Vaccination is progressing, with 50% of the population having received at least one dose, and a peak of 4.06% of the population receiving their first dose during WK23 (7.6-13.6)^[14]. An emerging concern is that the proportion receiving a first dose declined in WK24 and WK25 in Switzerland, with a 12% drop and a 34% from the peak, respectively.  A further rapid deceleration can be expected during the school summer holiday period unless additional activities to increase uptake are implemented now.

Activities such as advertising campaigns, mobile vaccination units in underserved areas, flexible vaccination hours, vaccination without the need for an appointment and increased numbers of appointments at centres with waiting lists, could all help to increase the accessibility and uptake of vaccination.

2. Test Trace Isolate Quarantine (TTIQ)

The TTIQ strategy can be adapted and improved to increase its effectiveness in a situation with low COVID-19 case numbers, widely available options for testing, new digital notification technologies, such as the NotifyMe app and molecular tests to detect variants of concern.

2.1. Testing

Testing for SARS-CoV-2 is easier than ever before. Identifying infections in asymptomatic people through regular testing in businesses and schools, increased testing of contacts and of those with minimal symptoms means that contact tracing can start sooner. The symptoms of infection with the delta variant might differ from those with other variants, with more ‘cold-like’ symptoms.

The number of tests is <20,000 per day as of late, and lower than in April and May 2021, so there is plenty of capacity for increasing the number of tests.

All positive rapid diagnostic test results should be confirmed by PCR for two reasons: first to exclude false positives, which become more frequent as the incidence declines (low positive predictive value), even when test specificity is high. Second, a swab sample can be tested for the presence of mutations by PCR and can be sent for whole genome sequencing.

All positive test results should undergo immediate reflex PCR testing to detect mutations that are present in the delta variant. Even though there will be false positives (because some mutations are also present in other variants, rapid detection of a possible case of delta variant infection allows contact tracing to start more quickly (see 2.2).

All PCR-positive samples should be sent to a laboratory that conducts whole genome sequencing and results of phylogenetic analysis should be linked with epidemiological data about the likely location of exposure. Genomic surveillance gives the most accurate information about the places from which delta variant infections are being imported and can enhance information about countries with limited genomic surveillance. Whole genome sequencing also allows rapid detection of other new, potentially more serious, variants of concern. With the current low caseload there is sufficient capacity to conduct whole genome sequencing on all newly detected SARS-CoV-2 infections.

2.2. Contact tracing

There is sufficient capacity to conduct enhanced contact tracing for all people with newly diagnosed SARS-CoV-2 infection and to treat each of these new index cases as though they might be infected with the delta variant. Speed is of the essence in contact tracing of people infected with a variant of concern. If the diagnosis has been made on a result from a rapid test, intensive contact tracing can begin, even before results of reflex PCR testing or whole genome sequencing are received. The principles underlying contact tracing for COVID-19^[15] and recommendations to increase contact tracing capacity have been published^[16].

Enhanced contact tracing includes:

– Mapping the contacts and locations of all newly diagnosed cases. Determine which new cases are linked epidemiologically to an existing known case – this means that a transmission chain has been broken. An absence of identifiable links to an existing case means that the source of infection needs to be found and isolated.

– Forward contact tracing to find, test and quarantine people exposed to the new index case in the last 48 hours. Then, finding the close contacts of these contacts as quickly as possible.

– Backward contact tracing for 10 days to find the potential source of infection. The potential sources are identified from a list of high-risk events, adapted from guidance used in Japan^[17]. Contact tracers can ensure that as many people as possible at those locations are tested as quickly as possible.

– Proactive offer to help people identified as contacts take up the offer of vaccination, if necessary.

2.3 Digital proximity tracing

Use of the SwissCovid and NotifyMe apps will further speed up the identification of unknown contacts.

2.3.1 Prevention of imported SARS-CoV-2 variants of concern

Active measures at the border for detection of SARS-CoV-2 infection should aim to minimise the risk of new SARS-CoV-2 outbreaks arising from undetected imported infection, particularly from new variants of concern and from countries with limited capacity for genomic surveillance. The European football championships have been associated with increased numbers of COVID-19 cases caused by the delta variant, for example in Denmark and Finland^[18]. The origin and destination of incoming travellers should be able to be tracked. A travel history should be documented for all people with newly diagnosed SARS-CoV-2 infection and their contacts. The precautionary principle should be applied when the characteristics of a new variant of concern have not been fully determined (e.g. transmissibility, severity of disease, reduction in vaccine effectiveness against transmission, infection or disease). Measures to reduce arrival of people from countries in which the variant of concern is circulating widely, and enhanced measures for testing, and for isolation or quarantine on arrival can delay the introduction and allow time to develop appropriate measures for prevention and surveillance.

2.4 Additional public health measures as part of combination prevention

Continued adherence to mask-wearing in indoor spaces, increased indoor ventilation, maintenance of safe distances between people is required to reduce the risk of transmission of SARS-CoV-2.