Less than one-third of close contacts in the ACT have gone on to test positive for COVID-19 within a fortnight of their exposure over the last six weeks.
The figures will give fresh confidence a change to quarantine requirements this week will not significantly increase the number of COVID-19 cases in the community.
There were 18,579 close contact declarations made to ACT Health in the past six weeks, which are made when people are notified by household members or ACT Health they are a close contact.
Within seven days of exposure, 5361 people declared a positive rapid antigen test result or had a positive PCR test notified to ACT Health, just under 29 per cent of declared close contacts in the period.
A further 471 people received a positive COVID-19 test result in the second week after declaring as a close contact, 2.5 per cent of declared close contacts.
ACT Health received 7140 notifications of negative PCR results from the cohort of close contacts within 14 days of their exposure.
The number of close contacts declared to ACT Health in the six-week period is likely lower than the total number of close contacts in the community, given there were more than 40,000 cases of COVID-19 reported in the six-week period.
A spokesman for ACT Health said the household contacts of people with COVID-19 were still at a higher risk of catching the virus than other people in the community.
"[That] is why upcoming changes to quarantine requirements contain significant risk-mitigation steps, such as regular testing, mask-wearing, avoiding high-risk settings, and only leaving the home if you are symptom-free," the spokesman said.
From 11.59pm on Tuesday, close contacts will be released from quarantine and will be able to go to work if they wear face masks when in public indoor spaces, have no COVID-19 symptoms and return regular negative results on rapid antigen tests.
The data from ACT Health is in line with recent research which analysed the attack rate of COVID-19 in households.
A study of almost 12,000 Danish households, which is yet to be peer-reviewed, found 31 per cent of household contacts of confirmed Omicron COVID-19 cases tested positive within seven days. The study found increased transmission was likely among unvaccinated individuals.
The study also found the secondary attack rate was 21 per cent among households with Delta infections of COVID-19. The Delta variant of COVID-19 has not been detected in the ACT since January.
However, a Centre for Disease Control and Prevention study in the United States found 52.7 per cent of household contacts of confirmed Omicron cases contracted COVID-19. That rate fell to 42.7 per cent among household contacts who had received a booster vaccination dose.
The attack rate was 39.5 per cent among household contacts who wore a mask at home at any point during the potentially infectious period of the index case, a 12-day period that started two days before a positive test result.
Meanwhile, health authorities in the ACT have confirmed about 40 per cent of future COVID-19 cases in the territory will go unconfirmed by tests for the virus.
But short-term modelling on case numbers predicted stable cases over the coming weeks that were generally in line with current daily case counts.
The ACT posted a decline in the number of daily COVID-19 cases reported on Sunday, as the territory inched towards 100,000 total infections since the start of the pandemic.
There were 725 new COVID-19 infections reported in the ACT, which included 376 positive PCR test notifications and 349 positive rapid antigen test results. No deaths were reported.
Forty-seven people died with COVID-19 up to the week ending April 17, but only four of those had received three doses of a COVID vaccination.
The proportion of people aged 16 and over in the ACT who have had three doses of a COVID vaccination - 74.8 per cent - has risen by just 1.5 percentage points since the start of April. Almost 97 per cent of the population aged five and up has had two doses.
Twenty-three people who died had received two doses of the vaccine, three had received one dose, while 16 of the people who died were unvaccinated.
"A COVID-19-related death is reported if the person dies with COVID-19, though it may not be the cause of death. Deaths under investigation by the coroner will not be reported until the findings have been issued," ACT Health said in its weekly epidemiological report.
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