📄 infectious diseases

Periodic intensification of routine immunization (PIRI): modeling a novel strategy to supplement routine and pulsed measles vaccination

This study uses a stochastic age-structured SIR model to demonstrate that Periodic Intensification of Routine Immunization (PIRI) can outperform or match Supplemental Immunization Activities (SIAs) in reducing measles cases, particularly in settings with intermediate routine immunization coverage or those experiencing continuously increasing coverage.

Original authors: Zou, K., Ferrari, M.

Published 2026-02-15

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Zou, K., Ferrari, M.

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

Imagine your community is a fortress trying to keep out a very contagious, mischievous ghost called Measles. To keep the ghost out, you need a strong wall of immunity.

For a long time, health workers have used two main tools to build this wall:

Routine Immunization (RI): This is the "daily bread." It's the regular, scheduled visits to the local clinic where parents bring their babies for shots. It's steady, but sometimes people miss it because they are busy, live far away, or don't trust the system.
Supplemental Immunization Activities (SIAs): These are the "big rescue parties." Every few years (say, every 5 years), the whole community shuts down for a massive campaign. They go door-to-door, set up tents in the town square, and vaccinate everyone in a wide age range, regardless of whether they've had shots before. It's powerful, but it's expensive, exhausting, and there's a long gap between parties where the ghost can sneak back in.

The New Idea: Periodic Intensification of Routine Immunization (PIRI)

The authors of this paper asked: "What if we didn't wait for the big rescue party every 5 years? What if we just made the daily bread delivery much more intense, but more often?"

Enter PIRI. Think of PIRI as a "Super Clinic Week." Instead of a massive, expensive door-to-door campaign, health workers take their existing clinics and supercharge them for a short period (like one month a year). They work harder, stay open longer, and focus specifically on the kids who missed their regular shots.

The Big Experiment: The "Rate Ratio" Race

The researchers built a computer simulation (a digital twin of a real population) to see how these three strategies compare:

Just the Daily Bread (Routine only).
Daily Bread + Big Rescue Parties (Routine + SIAs).
Daily Bread + Super Clinic Weeks (Routine + PIRI).

They wanted to find the "Magic Multiplier."

Question: If the Super Clinic Week (PIRI) happens once a year, how much harder do the health workers have to work compared to a normal week to beat the Big Rescue Party (SIA)?

The Surprising Findings: The "Goldilocks" Zone

The results created a funny "U-shaped" curve, which is like a slide at a playground.

1. The Low Slide (Low Routine Coverage):
If the regular clinics are doing a terrible job (very low coverage), the "Super Clinic Week" has to work impossibly hard to catch up. It's like trying to fill a leaking bucket with a teaspoon while the Big Rescue Party uses a firehose. In this scenario, the Big Rescue Party (SIA) is still better because the gap is just too huge.

2. The Goldilocks Zone (Medium Routine Coverage):
This is where the magic happens. If the regular clinics are doing a decent job (covering about 25% to 70% of kids), the "Super Clinic Week" wins easily!

The Magic Number: The health workers only need to work about 4 to 5 times harder than usual for that one month to beat the Big Rescue Party.
Why? Because PIRI is "smart." It targets the specific kids who missed the regular shots. The Big Rescue Party is "dumb" in a way—it wastes time vaccinating kids who are already safe, just to make sure it didn't miss anyone. In the Goldilocks zone, PIRI is more efficient and stops the ghost from building up a big army of susceptible kids between campaigns.

3. The High Slide (High Routine Coverage):
If the regular clinics are already doing an amazing job (covering 90%+ of kids), the Big Rescue Party wins again, but only by a tiny, tiny margin.

Why? At this level, the only kids left who are sick are the ones whose bodies didn't react to the vaccine (a "vaccine failure"). The Big Rescue Party catches these kids because it doesn't care about their history. PIRI, by definition, usually only looks for kids who never got a shot.
The Good News: Even though the Big Rescue Party wins here, the difference is so small that sticking with PIRI is still a great idea. It keeps the case numbers low and steady, preventing the scary, massive outbreaks that happen right before a Big Rescue Party.

The "Smooth Ride" vs. The "Rollercoaster"

The most important visual in the paper is about stability.

The SIA Strategy (Big Rescue Parties): Imagine a rollercoaster. You have a long, flat track where cases are low, then suddenly a massive spike (an outbreak) happens because the gap between parties was too long. Then the party comes, and the line drops to zero. It's a bumpy, scary ride.
The PIRI Strategy (Super Clinic Weeks): Imagine a smooth, gentle highway. Because you are checking in every year (or more often), you catch the kids who missed the shot before they can get sick and spread the virus. There are no massive spikes. The road is smooth.

The Takeaway

This paper suggests that for many countries, especially those where regular clinics are doing a "okay" job, we should stop waiting for the expensive, exhausting "Big Rescue Parties" every 5 years.

Instead, we should intensify the regular clinics once a year. It's like upgrading your weekly grocery run to a "Super Shopping Day" once a month. It's cheaper, it builds a stronger local health system, and it keeps the measles ghost from ever getting a foothold.

In short: Don't wait for the firehose every 5 years. Just turn up the hose on your regular sprinklers a little bit, more often, and you'll keep the garden safe.

1. Problem Statement

Measles control in low- and lower-middle-income countries relies on a dual strategy: Routine Immunization (RI) and Supplemental Immunization Activities (SIAs).

RI is delivered continuously at health centers but often suffers from low coverage due to accessibility issues, logistical barriers, and lack of trust. Global RI coverage has plateaued around 85%, falling short of the WHO's 95% target required for elimination.
SIAs are large-scale, pulsed campaigns (every 2–5 years) designed to reach children missed by RI. While effective at closing immunity gaps, they are labor-intensive, expensive, and non-selective (vaccinating already immune children). Furthermore, the long intervals between SIAs allow susceptible populations to accumulate, leading to outbreaks if RI coverage is insufficient.
The Gap: Periodic Intensification of Routine Immunization (PIRI) is a strategy that intensifies RI services over short periods (e.g., annually) using existing infrastructure. While PIRI has shown promise in cross-sectional studies (e.g., in India and Africa), there is no quantitative, mathematical framework comparing its efficacy against the standard "RI + SIA" approach. This study aims to fill that gap by modeling PIRI to determine if it can outperform or replace SIAs under various conditions.

2. Methodology

The authors developed a stochastic, age-structured Susceptible-Infected-Recovered (SIR) compartmental model to simulate measles transmission dynamics.

Model Structure:
- Population: 136 age groups (60 monthly groups for ages 0–59 months; 76 yearly groups for ages 5–80+).
- Compartments: Includes Maternal immunity (M), Susceptible (S), Infected (I), Recovered (R), and Vaccinated/Protected (P).
- Vaccination Tracking: The model distinguishes between individuals with recorded vaccination history (RI/PIRI) and those vaccinated via SIAs (where doses are not recorded, leaving them eligible for future RI/PIRI doses).
- Dynamics: Simulated using a discrete time-step (1 day) via the tau-leaping algorithm. It incorporates birth/death rates, waning maternal immunity (6 months), infection, recovery (8 days), and seasonal transmission (peaking in June).
- Parameters: Basic reproduction number ( $R_0$ ) set to 16. Vaccine efficacy assumed at 95%.
Vaccination Scenarios Simulated:
1. RI Only: Continuous vaccination with first and second doses.
2. RI + SIAs: RI plus a campaign every 5 years targeting children 9 months to 5 years old with 70% coverage.
3. RI + PIRI: RI plus an annual intensification period (e.g., 1 month) where vaccination rates are temporarily increased. PIRI is modeled as selective (targeting only unvaccinated/0-dose children) or less selective (targeting 0- and 1-dose children).
Key Metric: Minimally Sufficient Rate Ratio (MSRR)
- The study defines the MSRR as the ratio of the PIRI vaccination rate to the peak RI vaccination rate required for the "RI + PIRI" strategy to achieve equal or fewer measles cases than the "RI + SIAs" strategy.
- The authors scanned various RI coverage levels (0–100%) to identify the MSRR curve.

3. Key Contributions

Mathematical Formalization of PIRI: This is the first study to formally define PIRI within a dynamic transmission model, moving beyond descriptive case studies.
Operational Metric (MSRR): Instead of relying solely on coverage percentages (which are hard to measure in real-time for pulsed campaigns), the authors propose the Rate Ratio (PIRI rate vs. RI rate) as a practical, observable metric for program managers to gauge the necessary intensity of a PIRI campaign.
Strategic Transition Framework: The study provides a decision-making framework for countries to transition from SIA-dependent strategies to PIRI-based strategies based on their current RI coverage levels.

4. Key Results

The relationship between RI coverage and the effectiveness of PIRI vs. SIAs follows a U-shaped curve regarding the MSRR:

Low RI Coverage (<10%): PIRI cannot outperform SIAs. The required PIRI rate ratio becomes prohibitively high (approaching infinity) because the baseline RI rate is so low that a short PIRI window cannot vaccinate enough of the massive susceptible population to match the broad reach of a 5-year SIA.
Intermediate RI Coverage (25% – 70%): This is the "sweet spot" for PIRI.
- The MSRR is low, typically around 4 to 5 times the peak RI rate.
- In this range, an annual PIRI campaign achieves fewer total cases than a 5-yearly SIA.
- PIRI is more effective here because it prevents the accumulation of susceptibles between long SIA intervals.
High RI Coverage (>75%): SIAs regain the advantage.
- At high coverage, the remaining susceptible population consists largely of children who received vaccines but failed to seroconvert (primary vaccine failure).
- Since PIRI is defined as selective (targeting only unvaccinated children), it misses these vaccine failures. SIAs, being non-selective, vaccinate everyone in the age range, including those who need a booster or failed seroconversion, thus performing better.
- Note: The performance gap narrows significantly at high coverage, and switching PIRI to be less selective (targeting 1-dose children) can improve PIRI performance in high-coverage settings.
Dynamic Scenarios (RI Improvement over Time):
- Simulations of RI coverage increasing linearly from 0% to 100% showed that maintaining PIRI throughout the transition results in lower cumulative cases and smoother incidence trajectories compared to switching between SIAs and PIRI.
- SIA-based strategies exhibited sharp surges in cases following campaign lulls, whereas PIRI strategies maintained consistent suppression of outbreaks.
Sensitivity Analysis:
- Birth Rate: Higher birth rates favor PIRI, as the annual cadence prevents rapid susceptible accumulation better than 5-year SIA intervals.
- Selectivity: Broadening PIRI to include 1-dose children widens the range of RI coverage where PIRI is superior.
- Timing: Conducting PIRI/SIAs out of phase with the seasonal transmission peak improves efficacy.

5. Significance and Implications

Policy Guidance: The study suggests that countries with intermediate RI coverage (25–70%) should consider transitioning from expensive, infrequent SIAs to more frequent, intensified RI (PIRI).
Cost and Sustainability: PIRI leverages existing health infrastructure, potentially reducing the "surge" costs and logistical burdens of SIAs. It also helps build long-term system resilience and reduces "campaign fatigue" in communities.
Outbreak Prevention: By shortening the interval between vaccination opportunities (from 5 years to 1 year), PIRI is more effective at preventing the large-scale accumulation of susceptibles that drives major measles outbreaks.
Future Research: The authors note that while PIRI shows promise, a formal cost-effectiveness analysis is needed. They also highlight that PIRI must be optimized to avoid diverting resources from core RI services.

In conclusion, the paper demonstrates that PIRI is a viable, and often superior, alternative to SIAs for measles control, provided the country's routine immunization coverage is in an intermediate range and the PIRI campaign is intensified sufficiently (approx. 4–5x the normal RI rate) to target unvaccinated children effectively.