Early diagnosis of new symptomatic COVID-19 (SARS-CoV-2) cases

By Dr Nigel Raymond

29 April 2020

Elimination strategy context

NZ has completed a period of national ‘lock-down’, and is now stepping down from Level 4 to 3 controls, as part of the government’s stated elimination strategy. The number of daily reported cases is presently less than 10 daily, mostly associated with known clusters. A continuing small number of cases is expected due to returning travellers such as repatriating nationals, and it will be essential that quarantine arrangements are robust. The Ministry is completing its surveillance plan, supporting continuation of high levels of testing and scaling up contact tracing capacity. As of 27 April there is a combined total of 1,469 confirmed and probable cases, including 19 deaths for an overall crude mortality of just over 1%. The mortality risk has been substantially more for older adults, with those over 70 years accounting for most deaths, often associated with comorbidities and clusters in aged residential care.

Importance of early case detection

Early detection of the hopefully very small or even rare number of new cases will be an essential component to NZs elimination strategy over the coming months1.The viral load on upper respiratory tract sample PCR testing peaks within the first week of COVID-19 infection onset2. Early detection allows Public Health services to contain any cluster when it is smaller, less geographically spread and more readily contact traced. Wider outbreaks require greater contact tracing capacity or could otherwise warrant reintroduction of higher Level restrictions1.

Clinical Presentation

The median incubation period has been estimated to be 5.1 days, with only about 1% developing symptoms after 14 days3.

Symptomatic COVID-19 usually presents as a respiratory infection syndrome, most commonly with fever and cough4. According to two large studies, fever was reported in 82 – 87% of these cases. Other common symptoms of COVID-19 are cough, dyspnoea, fatigue, anorexia, anosmia, myalgia and acute confusion. Symptoms reported much less frequently (<5% of cases) include sore throat, rhinorrhoea, headache, chest pain, dizziness, abdominal pain, diarrhoea and nausea. The frequency of symptoms is derived from overseas experience, where testing has sometimes been directed to those with more severe illness5. There is currently a lack of NZ data on presenting symptoms. 

Which symptomatic patients to test for COVID-19

The current NZ suspect case definition includes those with symptoms of an acute respiratory infection with or without fever. Testing can also be carried out with less common symptoms when the clinician suspects COVID-19, such as with gastrointestinal symptoms and a history of exposure. 

The optimal approach to testing of adults and children will continue to evolve to meet clinical and public health needs, and should be guided by emerging evidence6. The low number of new reported cases and recent community surveys suggest a present likely low population prevalence. In this setting, the pre-test likelihood of people presenting with COVID-19 like symptoms is expected to be low (e.g. <1%) in those without a significant exposure history. Most value will be attained by testing those with common COVID-19 symptoms (e.g. >5-10%). Usually, the incidence of influenza-like illness (ILI) due to influenza and other common respiratory viruses increases over winter, and it will be interesting to see whether non-COVID ILI will be less this year following social distancing and public restrictions. 

While the large majority of suspect cases without a significant exposure history will not have COVID-19, it will still be important to test given the community consequence of not diagnosing a case is high. Clear communication about the purpose of testing for COVID-19 and removal of barriers to prompt testing will be essential. 

Possible exposure risk in symptomatic people

COVID-19 cases to date have mostly occurred in returned travellers and their contacts, and contacts of secondary cases and clusters. The product of exposure proximity and duration is a useful guide to the chance of acquiring infection. Some essential workers may have an increased risk, due to contact with more people outside their ‘bubble’. Healthcare workers have comprised 9% (to 16 April) of NZ cases, mostly associated with clusters. Testing symptomatic HCW will contribute to ensuring healthcare facilities are safe for both staff and patients. Understanding the nature of exposure risk over the coming months will be informed by the results of clinical and surveillance testing. It will remain essential to attend to each person’s more likely diagnosis and needs, while also testing to exclude the often small possibility of COVID-19. 

Probable and confirmed cases: management and clinical advice

Useful resources for clinical management guidance can be found at the Ministry of Health, ASID, and Australian Clinical COVID-19 Evidence Taskforce websites.

Clinical deterioration sometimes occurs during the week following diagnosis, and hospitalization has occurred on average a week following symptom onset in overseas case series7.Reliable predictors of progression are as yet not well validated for COVID-198.Advance discussion of the goals of care can be valuable for those with advanced age, frailty and comorbidity. Clinical signs of organ dysfunction e.g. hypotension, tachipnoea, hypoxia, confusion, raised creatinine are likely to predict deterioration as for sepsis with other infections. Measurement of vital signs and pulse oximetry, while using appropriate PPE, is likely to be more reliable than telemedicine assessment. 

In summary, while it is hoped that new cases of COVID-19 infection in NZ are few, priority should be given to diagnosing cases early as part of the elimination strategy.


  1. Verrall A. Rapid audit of contact tracing for Covid-19 in New Zealand. https://www.health.govt.nz/system/files/documents/publications/contact_tracing_report_verrall.pdf
  2. Liu Y, et al. Viral dynamics in mild and severe cases of COVID-19. Lancet Infect Dis 2020. Published Online March 19, 2020. https://doi.org/10.1016/S1473-3099(20)30232-2
  3. Lauer SA, et al. The incubation period of coronavirus disease 2019 (COVID-19) from publicly reported confirmed cases: estimation and application. Ann Intern Med. doi:10.7326/M20-0504
  4. Thevarajan I, Buising KI, Cowie BC. Clinical presentation and management of COVID-19. Med J Aust 2020;  https://www.mja.com.au/journal/2020/clinical-presentation-and-management-covid-19 [Preprint, 8 April 2020]
  5. Murdoch DR, Weiss P. Clinical course and mortality risk of severe COVID-19. https://doi.org/10.1016/S0140-6736(20)30633-4
  6. Ludvigsson JF. Systematic review of COVID-19 in children shows milder cases  and a better prognosis than adults. Acta Paediatrica https://doi-org.wmezproxy.wnmeds.ac.nz/10.1111/apa.15270
  7. Huang C, et al. Clinical features of patients infected with 2019 novel coronavirus in Wuhan, China. Lancet 2020, Feb 15, 395(10223): 497-506. https://els-jbs-prod-cdn.jbs.elsevierhealth.com/pb/assets/raw/Lancet/infographics/coronavirus/Coronavirus_MedianTimeline_Infographic-1584612208650.jpg
  8. Wynants L, et al. Prediction models for diagnosis and prognosis of covid-19 infection: systematic review and critical appraisal BMJ2020;369:m1328 http://dx.doi.org/10.1136 bmj.m1328