The CHD Ploen is one of 15 health departments within the federal state of Schleswig-Holstein (2.8 million). Ploen County (population 128,686) is both a spread-out countryside (59% of the population) and a congested municipal residential area adjacent to the town of Kiel (245,000) with about the other 41% of the population. End of November 2020, Ploen county had a cumulative incidence in the second wave of around 130 per 100.000 and the city of Kiel of 330 per 100.000. There are surely several reasons contributing to this difference which cannot be further addressed here than just to mention that Hh-Q2° is not used in the CHD Kiel.
The first wave started in our region at the beginning of March, 2020. As of 9 March and enhanced at 23 March, lockdown measures were implemented and maintained until the first week of June, 2020. Already on 17 March, the epidemic curve had started to flatten. During the summer, more and more restrictions were lifted and even big events were allowed. In August and September, travel- associated issues were on the forefront until the first week of October, when the second wave set in. As of 2 November, a ‘lockdown light’ was implemented until 16 December when another strict lockdown was ordered.
Exposure and exposure measurement by contact tracing as a form of personal interview are key in field epidemiology . How many persons to be put into quarantine depends obviously upon several factors such as contact pattern, intensity of contact, time axis, strategy towards direct and indirect contacts, societal structure, i.e. fraction of single household and household size, and particularly on the precision of the work of a CHD. The indication for Hh-Q2° was triggered by a delay of notification of the CHD by at least 72 h (60–96 h). This is twice the range of the minimum latency period of SARS-CoV2 (Fig. 1) [2, 7, 12]. Obviously this time window would need more modelling based on a larger sample size, but according to our practical experience, it seems to work. PCR testing at decision points in contact tracing and for separation within households is of great value.
The fraction of single households in our cohort was 29.8% (111 out of 372) and mirrors the societal structure with fewer families and children in general. With 2.1 (1°contacts only) and 2.9 (1° and 2°contacts) persons per primary case put into quarantine, this should be an acceptable burden for society and is much less than that modelled by Aleta et al.  and Hinch et al.  or in the other CHDs in our region.
The efficacy of Hh-Q2° with 51.5% of the efficacy of quarantine in 1°contact was surprisingly high and is obviously influenced by the quality of the investigations by the CHD and the number of Hh-Q2° ordered (the denominator). Household size appears to be a major risk factor for conversion of contacts into cases or ascertainment of converted contacts as described also by a seroprevalence study in Sweden . Adolescents and young adults were playing an increasing role in the second wave as 1°contact to households with further members according to the “heat chart” of age-specific attack rates over time . Persons in this age group to a large extent still live in the parental home with the original family.
In this study, 16.4% of all converted and symptomatic cases in quarantined persons were ascertained via Hh-Q2°. This means that 1 in 6 cases were additionally prevented from spreading the infection further within the community. The all-over impact of Hh-Q2° detecting and containing 5.9% of all cases (21 out of 353) seems large enough to justify the effort in ordering quarantine for 2°contacts.
Hh-Q2° to prevent tertiary cases used early in an outbreak or a pandemic wave can make an impact and increase the efficiency of NPI. The early seeding of chains of infection can be prevented by Hh-Q and makes the virus to run into a dead-end. Hh-Q2° on a comprehensive scale is the preferable option in contrast to a lockdown of the general population. With Hh-Q2°, a lockdown might be prevented or at least significantly delayed as also assumed by Aleta et al. . To further justify Hh-Q2°, investigations within backward contact tracing must be as accurate and as rapid as possible to tailor the quarantine orders, including Hh-Q2°, only to the fraction of the contact pattern in which exposure is most likely. Again this depends upon the quality and efficiency of the work of the CHD. In spite of using Hh-Q2° as a tool in the CHD Ploen, the ratio of cases to quarantine orders was lower than in other CHDs in the region. The ultimate goal is to raise ɛ in spite of the counterproductive viral characteristics (Fig. 1). The manifestation index is subject to the virus-host interaction; the ascertainment of cases in the population in general depends upon the degree of testing; but the management of quarantine and the use of Hh-Q2° are under the authority of the CHD.
In analogy to Brockmann and Helbing , the spread at the local level and even in the household setting can be regarded in the same way (Fig. 3). The close contact would be the first wave, the hub, knocking on the door of the non-case household. If the household is not stratified in time, the incubation of the entire household or setting continues in case the contact 1° starts to shed and evolves into a case. The latter can only partially be identified in time, given the key parameters of SARS-CoV2 such as θ including the fraction asymptomatic but infectious subjects.
In general, the household as an entity and endpoint of public health considerations has so far only been partially recognized and accepted, since our health care thinking is to far extent focused on individual aspects. John Oxford [Vienna conference “Influenza Vaccines for the World”, 18 to 20 October 2006] pointed out for the first time and on many occasions thereafter that the 1918 pandemic (“Spanish Flu”) was primarily a tragedy of families. Once the virus entered a family, the death toll was significant. The virus enters into families via one family member (a 1°contact) having had an efficacious contact outside. In analogy to the Japanese cluster approach for backward contact tracing , Hh-Q2° could be seen as the equivalent forward-orientated control approach.
All measures taken within the bundle of NPI also have to be seen in the context of compatibility with social aspects and thus raising acceptability and compliance. Ordering Hh-Q, at least due to the current regulation in Germany, is synergistic, since parents with children under quarantine, for example, do not have to bother about sick-leave or any other option to justify staying at home, since a quarantine order entitles them to social security and compensatory salary payment. The main argument, however, remains the public health intervention and blocking the chain of transmission. The conversion of the 1°contact into a case with shedding at least two days before symptom onset or as an asymptomatic spreader is the cornerstone of the argument for Hh-Q2°. According to our observations, about one in 10 tertiary cases occurs without symptoms in the close contact of that household.
The inability and time delay of detection of this conversion with onset of viral shedding caused by the fraction θ including the fraction of asymptomatic, are facts and surveillance of 1°contact by health departments is necessarily inefficient due to the time and shedding characteristics of SARS-CoV2. The most recent data of Zhang et al.  makes this effort appear even more inefficient since they pointed out that even with tight testing of household members in quarantine, many are missed as their serological data revealed. Furthermore, the numbers in quarantine increase rapidly in a pandemic wave and the manpower bound by active surveillance accumulates accordingly. This manpower could better be used otherwise, for instance in ambulatory testing of clusters.
Since incubation of the household continues over the entire period of a 1°contact, if it starts to shed virus, the current 14 days of quarantine are supportive for the efficacy of Hh-Q2°. A shortening of the quarantine duration from 14 to 10 days could have a detrimental impact, since the 14 days so far, guaranteed to most extend that 2°contacts would still be in quarantine at the time of being transmissible, whether symptomatic or not, after being infected by a 1°contact.
Finally, all this has to be driven by the motivation to contain or flatten the pandemic wave to protect the vulnerable but still limiting the burden for the general society as much as possible. The tool of Hh-Q2° is easy to order and logical at the same time. It is astonishing that it was widely overlooked and not identified, at least by authorities, as a straightforward measure within the tool box of NPI. So far it has only been addressed by modellers [15, 16]. Across the entire sessions concerning SARS-CoV2 and COVID-19 during the ESCAIDE conference on 26 and 27 November 2020 organised by the ECDC, it was only mentioned on one slide within the keynote lecture by George Gao, head of the Centre for Disease Control China, in regard to lessons learned in China . As demonstrated within the study presented here, Hh-Q2° is also feasible in Western countries. Interestingly the concept of ring vaccination in the endgame of the smallpox eradication campaign was based on the same principle – in this case vaccination of the household members of 1°contact persons of a case (Adam Finn, 39th Annual Conference of the European Paediatric Infectious Diseases Society). We advocate giving Hh-Q2° a higher priority within the tool box of NPI, at least for the control of SARS-CoV2, as already reported by Aleta et al. . Whether it is more widely used than made public, remains an open issue. If explained to persons to be put under Hh-Q, it is widely accepted and plausible. Hh-Q2° is to a greater extent not yet addressed in national guidelines since it is a field approach and is easily overlooked by national authorities. The RKI should urgently integrate the approach demonstrated here into their national guidelines. The tool of Hh-Q2° is both logical and straightforward.