Disruption and recovery of notifiable infectious diseases after COVID-19 in Australia, 2015-2025

This study analyzes Australian surveillance data from 2015 to 2025 to reveal that while non-pharmaceutical interventions significantly suppressed 28 notifiable diseases, post-pandemic recovery has been heterogeneous and incomplete by 2025, with 15 diseases remaining below baseline levels, 17 exhibiting overshoot, and statistically confirmed immunity debt observed only for rotavirus.

The Gist

Imagine Australia's population as a giant, bustling city where tiny, invisible "germ travelers" constantly move around, causing different types of sickness. Before 2020, these travelers followed a predictable rhythm, like a clockwork machine.

Then, the world hit the "Pause Button" with the arrival of COVID-19. To stop the virus, the city locked its doors (border closures) and asked everyone to stay home (non-pharmaceutical interventions).

This paper is like a traffic report looking at what happened to 47 different "germ travelers" from 2015 all the way to 2025. It asks two big questions:

1. How much did the lockdowns stop the other travelers?
2. Now that the doors are open again, have they all come back to their normal schedules?

Here is the story of what the researchers found, broken down simply:

1. The Great Silence (2020–2021)

When the city locked its doors, the "germ traffic" didn't just slow down; it mostly stopped.

• The Effect: For 28 out of the 47 diseases, the number of cases dropped by about half. It was like a highway that suddenly had no cars on it.
• The Travelers Most Affected: The diseases that usually "fly in" from other countries (import-dependent) were hit hardest because the borders were closed. Also, diseases that rely on kids mixing together in schools (like some vaccine-preventable ones) went quiet because schools were closed or kids stayed home.

2. The "Immunity Debt" (The Missed Practice)

Think of our immune systems like muscles. If you don't exercise them, they get weak.

• The Theory: Because everyone stayed home and didn't get exposed to these germs for a couple of years, our "muscles" (immunity) got a bit lazy.
• The Result: When the doors opened, some germs came back with a vengeance, causing a "surge" or an overshoot. It's like a rubber band that was stretched tight and then snapped back too hard.
• The Reality Check: The researchers looked for this "snap back" in five specific diseases. They expected to see a massive spike (immunity debt) in all of them. However, the math only proved this happened clearly for one disease: Rotavirus (a stomach bug). For others, like the Flu, the numbers were wild and unpredictable, so they couldn't say for sure if it was a "debt" or just normal chaos.

3. The Uneven Recovery (2025 Status)

By 2025, three years after the restrictions eased, the city hasn't fully returned to its old rhythm. The recovery is messy and uneven:

• The Overachievers (17 diseases): These germs came back so fast that they are now causing more sickness than they did before the pandemic. They are running a marathon at full speed.
• The Normalizers (12 diseases): These have found their old pace and are behaving exactly as they did before 2020.
• The Stragglers (15 diseases): These are the ones that are still "stuck." Even though the doors are open, these germs haven't returned to their usual numbers. They are still suppressed, perhaps because the population has built up some natural protection or because the virus just hasn't found its way back in yet.
• The Mystery (3 diseases): The data for these is too messy to tell a clear story.

The Big Takeaway

The main lesson is that one size does not fit all.

You can't treat all diseases the same way.

• For the Overachievers, we might need to be extra careful and maybe get extra vaccines to stop the surges.
• For the Stragglers (the 15 that are still low), we need to keep watching them closely. They might be hiding, and if they suddenly wake up, they could cause a surprise outbreak.

In short: The pandemic paused the flow of many germs, but when the water started flowing again, some rivers flooded, some returned to normal, and some are still dry. We need to keep an eye on all of them, especially the ones that haven't woken up yet.

Technical Summary

Technical Summary: Disruption and Recovery of Notifiable Infectious Diseases in Australia (2015–2025)

1. Problem Statement

While the immediate impact of COVID-19 non-pharmaceutical interventions (NPIs) on suppressing infectious disease transmission is well-documented, there is a significant gap in understanding the post-pandemic recovery trajectories across diverse pathogens. Specifically, it remains unclear how different transmission modes, border dependencies, and vaccine-preventable mechanisms influence the speed and completeness of recovery. Furthermore, the phenomenon of "immunity debt" (susceptibility accumulation due to reduced exposure) and the subsequent risk of disease overshoot require systematic quantification. This study addresses the lack of longitudinal analysis covering the full spectrum of nationally notifiable diseases in Australia from the pre-pandemic baseline through the recovery phase (2015–2025).

2. Methodology

The study utilized a robust, multi-faceted statistical approach applied to surveillance data from the National Notifiable Diseases Surveillance System (NNDSS) covering 47 nationally notifiable diseases across six transmission modes.

• Baseline Definition: A pre-pandemic baseline was established using data from 2015–2019.
• Disruption Quantification: Disease disruption was measured using Observed-to-Expected (O/E) ratios, comparing 2020–2021 incidence against the established baseline.
• Causal Inference: A Difference-in-Differences (DiD) model was employed to isolate the causal effects of specific NPIs (specifically border closures) on disease suppression, distinguishing between import-dependent and domestic transmission dynamics.
• Recovery Analysis: Kaplan-Meier survival analysis was utilized to estimate the "time-to-recovery" for each disease, defining recovery as the return to expected incidence levels.
• Immunity Debt Assessment: The study calculated cumulative immunity debt and tested for statistically significant "overshoot" (incidence exceeding baseline) using bootstrap 95% confidence intervals.

3. Key Contributions

• Comprehensive Scope: This is one of the first studies to systematically analyze the disruption and recovery of a broad panel of 47 diverse pathogens rather than focusing on a single disease or class.
• Causal Attribution: By applying DiD analysis, the study moves beyond correlation to quantify the specific impact of border closures on import-dependent diseases.
• Trajectory Classification: The research introduces a nuanced classification of recovery states, distinguishing between diseases that have returned to baseline, those in partial recovery, those in sustained suppression, and those exhibiting overshoot.
• Statistical Rigor on Immunity Debt: It provides a critical statistical reality check on the "immunity debt" hypothesis, using confidence intervals to differentiate between observed trends and statistically significant excess incidence.

4. Key Results

• Initial Disruption (2020–2021):

  • 28 out of 47 diseases experienced a decrease in incidence, with a median O/E ratio of 0.51.
  • Border-sensitive and vaccine-preventable diseases were the most severely affected.
  • DiD analysis confirmed that border closures significantly suppressed import-dependent diseases (Coefficient: -0.50, 95% CI: -0.90 to -0.10, p=0.016).

• Recovery Status (as of 2025):

  • Heterogeneous Recovery: The recovery landscape is highly varied.
    • 17 diseases exceeded baseline levels (overshoot).
    • 12 diseases returned to expected levels.
    • 15 diseases remained below baseline (9 partially recovered, 6 in sustained suppression).
    • 3 diseases had insufficient data for classification.

• Immunity Debt and Overshoot:

  • Five diseases exhibited a "suppression-then-overshoot" trajectory suggestive of immunity debt.
  • Statistical Confirmation: Only Rotavirus showed a statistically significant cumulative excess (confirmed via bootstrap 95% CI).
  • Influenza: While showing a large absolute overshoot, high baseline variability prevented statistical confirmation of a significant excess.

5. Significance and Implications

The study concludes that post-pandemic recovery in Australia is incomplete and heterogeneous. The findings challenge the assumption of a uniform return to pre-pandemic norms and highlight two critical public health imperatives:

1. Differentiated Surveillance: Continued, targeted monitoring is required for the 15 diseases that remain suppressed, as they may represent emerging risks or require specific intervention strategies distinct from those for overshooting diseases.
2. Targeted Vaccination: The data supports the implementation of catch-up vaccination programs specifically for pandemic birth cohorts to address immunity gaps, particularly for vaccine-preventable diseases that have not yet recovered to baseline levels.

This research provides a data-driven framework for policymakers to move beyond generic pandemic recovery strategies toward pathogen-specific management plans.