A Quantitative Evaluation of the Impact of Vaccine Roll-Out Rate and Coverage on Reducing Deaths

Vaccination campaigns are often remembered by their final number: the share of a population that eventually received a complete course. That figure can conceal the weeks in which protection was still absent. During an epidemic, a dose given before a large wave can avert an admission or death. The same dose delivered after the wave has passed may still protect against the future, while arriving too late for the people already infected. Iran's COVID-19 campaign made this difference between coverage and timing measurable.

I was the fourth-listed author in a large collaboration that asked how many deaths might have been averted if Iran had vaccinated at the pace of other countries. By 20 April 2022, about 67.5 per cent of Iran's population was fully vaccinated. The analysis estimated 292,666 deaths associated with COVID-19 over the study period using excess mortality. That approach compares the total number of deaths with the number expected from earlier patterns, capturing some pandemic deaths that a laboratory-confirmed count may miss.

The central experiment took place inside a model. Researchers cannot send a country back through time and begin its vaccination programme on another date. We can construct a counterfactual, a carefully specified version of events in which one important feature changes. The team replaced Iran's per-person rollout rate with the rates observed in eight comparison countries that had relied mainly on inactivated-virus vaccines. Other elements of the Iranian epidemic, including its age structure and estimated mortality, remained part of the calculation.

An inactivated vaccine contains virus treated so that it cannot reproduce. It presents immune cells with recognisable material and can prepare the body to respond to later infection. Focusing on countries where these products predominated made the comparisons more coherent, although their health systems and epidemics still differed. The model countries were therefore scenarios rather than contestants in a ranking. Their rollout curves let the researchers ask what earlier or faster protection could have changed in Iran.

The answer depended sharply on time. Iran would have reached a broadly similar final level of full vaccination under the rates seen in Turkey, Bangladesh, Nepal or Sri Lanka. Their trajectories toward that level were different. Applying Turkey's faster rate to Iran was associated with an estimated 50,000 additional deaths averted, with a 95 per cent confidence interval from 38,100 to 53,500. Following Bangladesh's slower rate could have produced roughly 52,800 more deaths, although the uncertainty around that estimate was much wider.

This apparent contradiction dissolves when the epidemic curve enters the story. Turkey began earlier and moved faster, which would have protected more Iranians before the Alpha and Delta waves. Bangladesh reached a comparable overall coverage after more danger had already passed through the population. A final percentage treated both campaigns as equivalent. The model showed how many people were protected on the days when exposure was most consequential.

Age made the timing still more important. The risk of dying after infection rises steeply among older adults. A campaign that reaches them quickly can avert many deaths even if its later population coverage remains modest. In scenarios based on the faster rollouts of Montenegro or Bolivia, Iran could have protected more people over 50 during early waves despite ending with lower overall coverage. The later shortfall among younger adults became more important when Omicron spread. Speed and breadth served different moments of the epidemic.

The comparison with Argentina sharpened the point. Its scenario produced a higher final proportion of fully vaccinated people, close to 79 per cent, yet its slower rollout was associated with only about 12,600 additional deaths averted in Iran. Many of the doses accumulated after the Alpha and Delta waves had already taken their toll. Coverage remained valuable against later transmission, but it could not travel backward. The epidemic had placed an expiry date on part of the opportunity.

Bahrain provided an upper-bound comparison. If Iran had followed its rollout pattern, the model estimated that about 75,300 additional deaths might have been averted, with a confidence interval from 56,000 to 83,000. The size of that number should invite attention and caution in equal measure. It represents the output of assumptions applied to incomplete real-world data. It does not claim that Iran could simply have copied Bahrain's institutions or supply arrangements.

Protection itself was delayed after each injection. Immune cells need time to expand and mature, and a full primary course may require more than one dose. A dashboard can show a vaccination on the day it occurs while the person remains vulnerable for days or weeks. The model accounted for the relationship between rollout and mortality over time. Public communication should make the delay equally visible, especially when infections are rising and people may assume an appointment produces immediate immunity.

Every counterfactual depends on what it holds constant. Vaccine effectiveness can vary by age and by the viral variant in circulation. Prior infection changes population immunity. Public-health measures and behaviour influence who becomes exposed. The study varied important inputs and reported ranges around its estimates, yet no model can reproduce every interaction. Its strongest conclusion lies in the consistent direction across scenarios: faster delivery placed protection before more deaths.

Excess mortality introduced another layer of uncertainty while addressing a serious reporting problem. Deaths above the expected baseline may include indirect effects of disrupted care, and the baseline itself can be estimated in different ways. Confirmed COVID-19 deaths can miss people who were never tested. Using all-cause mortality allowed the analysis to look beyond limited case detection, provided the result was interpreted as an estimate of the pandemic's broader toll rather than a perfect count.

The study also turns attention toward the global supply system. A country's pace reflects national planning, but it also reflects purchasing power and access to manufacturers. Wealthier states secured large advance orders while others waited. Export restrictions or financing delays could shift a campaign by weeks, which the model suggests could carry a large mortality cost. Judging the rollout solely as a measure of domestic competence would overlook the international decisions that shaped when doses became available.

Once vaccines arrived, policy determined how quickly they became immunity. Prioritising older adults could produce a larger early reduction in deaths than distributing the same doses evenly. Clinics needed staffing and records capable of completing full courses. Public confidence and convenient access determined whether invitations became injections. A national dashboard showing cumulative doses could encourage attention to volume while obscuring the age groups still exposed and the slowing pace beneath the total.

Prioritisation also involved an ethical choice about which outcome the programme was trying to prevent first. Protecting those most likely to die could reduce mortality quickly. Reaching workers with many daily contacts might slow transmission and protect others indirectly. These goals can point toward different queues when supply is scarce. A transparent strategy states the goal, explains the evidence and revisits the sequence as coverage or the epidemic changes. Hidden reasoning makes every exception look like favouritism.

Better dashboards would keep time visible. They could show the rate of new protection and coverage within groups facing the highest risk, alongside the delay before immunity develops. A steep curve among older adults before a wave means something different from the same number of doses distributed slowly across many months. Metrics influence management because they determine which shortfall becomes obvious. The choice of what to display is part of the strategy.

The lesson extends beyond COVID-19. When a vaccine becomes available during an active outbreak, procurement speed and the sequence of delivery can change its benefit. High eventual coverage remains valuable, especially for lasting control. The model warns against pursuing that distant target while overlooking who remains unprotected today. A campaign has to protect quickly and continue reaching people after the initial urgency fades.

Iran's final coverage figure recorded a substantial national effort. It could not show the lives attached to earlier weeks. Counterfactual modelling gave those weeks a scale, even if the exact number remains uncertain. A dose is usually described by what it contains. During an epidemic, its value also comes from when it arrives. Public-health planning should treat time as part of the vaccine.