Coronavirus Australia: NSW, Sydney virus cases more than triple more than detected, study suggests

The number of first wave Covid cases in NSW was likely 3.5 times more than the cases actually detected, study suggests.

A nurse holds a COVID-19 swab test ready for processing at a Bondi Beach drive-through testing clinic on July 22, 2020. Picture: Lisa Maree Williams/Getty

There was an estimated triple the number of coronavirus infections in NSW compared with the number of cases detected, new research that analysed blood samples of pathology patients and blood donors has found.

A study into the seroprevalence of COVID-19 during the pandemic’s first wave in NSW has revealed that there were an estimated 7450 infections in the first wave of the pandemic in NSW, while only 2118 cases were officially notified following positive COVID-19 tests.

The study in the Medical Journal of Australia estimated that the true range of infections could range from 2118 to as many as 20,370, however it said that the estimated infection to case ratio the study reports should be interpreted with caution due to the small numbers of antibody positive specimens.

Of the blood samples analysed, 63 per cent of people were found to have neutralising antibodies to COVID-19.

The study analysed 5339 blood specimens from public and private pathology laboratories and Red Cross Lifeblood. Of those specimens, 38 were positive for COVID-19, including 0.15% of people who had a general pathology blood test, 0.79% for women aged 20-39 years undergoing antenatal screening, and 0.69% for blood donors.

Estimate exceeds 20,000

“Based on the seroprevalence point estimate of 0.15% from the general pathology collection covering all age groups, the estimated cumulative number of SARS-CoV-2 infections in the Sydney population of five million people was 7450, giving an estimated infection to case ratio of 3.5 when comparing to the number of notified cases up to 30 April 2020 (2,118), with a plausible range from 2118 to 20,370 based on the credible interval around the 0.15% estimate,” the study’s authors found.

Despite the estimate that there were 3.5 infections with SARS-CoV-2 for every officially notified case of COVID-19, the authors concluded that there was only limited community transmission of the disease in NSW.

‘Limited community transmission’

“Our study provides robust evidence that there was limited community transmission during the first epidemic wave of COVID-19 in Sydney,” the study’s authors found. “This is undoubtedly due to the early and successful implementation of national and state-based public health measures, including rapid upscaling of capacity to test and contact trace, strict border controls and quarantining of overseas travellers, movement and mixing restrictions, and a high degree of compliance with these measures by the public.

NSW authorities continue to urge people with mild symptoms to get tested. Picture: Lisa Maree Williams/Getty Images

“These findings indicate early and successful control of COVID-19, but also highlight the need to maintain efforts to mitigate further transmission.”

There have been 4,285 cases of COVID-19 officially reported in NSW since the pandemic began. There has been concern that a significant proportion of cases could be asymptomatic and community transmission could therefore be occurring undetected, and the MJA study indicates that this is to some extent true.

Health authorities have consistently urged anyone with even the mildest of symptoms to get tested for COVID-19. A total of 2,186,874 COVID-19 tests have been carried out in NSW. The peak number of cases of COVID-19 occurred on March 28 when 469 cases were diagnosed. Case numbers rapidly declined nationally after that point as restrictions were imposed and the first wave was brought under control, apart from a devastating second wave in Victoria.

The MJA study found that 20-29-year-old blood donors had a higher number seropositive than other age groups and there were no seropositive children under 10 years old.

“Our results are broadly consistent with the only other Australian serosurvey to date, which examined adult elective surgery patients from selected hospitals in four states admitted in June-July 2020,” the authors said. “The results of these two Australian serosurveys contrast with estimates from many countries in Europe, Asia (particularly India) and the Americas where pandemic control has been less effective, and reported seroprevalence has exceeded 10 per cent.”

“Reported COVID-19 cases represent an underestimate of the true number of infections in the population. The main reason is that a proportion of people with SARS-CoV-2 infection do not develop symptoms, or have mild symptoms, so do not attend clinical services for virus testing and diagnosis. Others do not access testing when unwell. As a consequence, these infections are missing in case-based reporting.”