Methods
All individuals with locally-acquired confirmed SARS-CoV-2 infection, reported in the NSW Notifiable Conditions Information Management System (NCIMS) referred to here as ‘COVID-19 cases’ to 8pm on 7 October 2021 with a disease onset date from 16 June 2021 were extracted.
Vaccination status was initially determined based on what was reported in NCIMS (this information is collected at interview or on case review by the public health response teams).
Vaccination data was then enhanced by matching to an extract of vaccination status from the Australian Immunisation Register (AIR).
The AIR data was extracted on 7 October 2021 and included all people who were NSW residents and had any record of a COVID-19 vaccine. Individuals from the AIR extract were matched to NCIMS cases using an exact match on first name, last name and date of birth.
If there were data on vaccination from both sources, AIR data was used as the source of information for analysis. Where data on vaccination existed in one system only, data from that system was used, if it was sufficiently complete. In the analyses, vaccination status was classified based on the following definitions:
• None: Reported as either: a) no vaccine record in NCIMS and unable to link to a record of any COVID-19 vaccination in AIR, or b) interval from receipt of first vaccine dose was too short to be effective (<21 days);
• One: One dose of vaccine at least 21 days prior to onset date;
• Two: Two doses of vaccine with the second dose at least 14 days prior to diagnosis and a minimum of 14 days between the two doses;
• Unknown: Information on vaccination (or vaccination date) unknown in NCIMS and unable to link to an AIR record of COVID-19 vaccination.
For analyses of rates of infections (described below) the group who had received one dose of vaccine, but the interval from vaccine receipt to case diagnosis was too short to be effective (<21 days), were classified separately from those not vaccinated.
The rate of infections amongst vaccinated and unvaccinated populations was estimated in fortnightly intervals since the outbreak began (to 30 June, 14 July, 28 July, 11 August, 25 August, 8 September, 22 September and 6 October) and by age group.
For this estimate, the population was restricted to those aged 12 years and older and resident in the Greater Sydney region based on Statistical Local Area.
Vaccination coverage data were extracted at the median date in each fortnightly interval. Rates of infection in the unvaccinated population were estimated by dividing the number of confirmed cases who had not received a COVID-19 vaccine, or who had unknown vaccination status, by the number of individuals in AIR reported with no doses in each interval.
People with unknown vaccination status were included in the no vaccine category following a detailed review of the AIR records of a sample (see below). Rates of infection in those with 2 doses of COVID-19 vaccine were estimated in a similar manner although taking into account a 14-day interval since receipt of dose 2 of the vaccine for it to be effective.
To determine the presence of comorbidities or residence in aged care among COVID-19 cases who were admitted to ICU or died and had two doses of vaccine, health records were manually reviewed.
