Tdevelopment of covid-19 vaccines was a scientific triumph. Converting it into medical benefits has required getting shots into the arms of as many people as possible, a process fraught with political and logistical hurdles. Nevertheless, a new study finds that jabs cut the disease’s death toll by 63% during their first year.
To calculate the number of lives saved by vaccines, researchers at Imperial College London needed to estimate how many people would have died without the jabs. They simulated this scenario using an epidemiological model, which calculates the shares of the population that are susceptible to a disease; get exposed; get infected; die or recover; and are vulnerable to reinfection. Each of these rates depends on factors like the transmissibility and severity of the pathogen, how much people interact with each other, demography, health-care quality and, crucially, vaccination.
Such models can simulate any disease. The authors customized theirs for covid by finding the mix of impacts of these variables that yielded the best predictions of deaths in each country in each week. Because official tallies tend to undercount mortality caused by covid, the study related on The Economist‘s model of excess deaths, which estimates how many more people have died of all causes during the pandemic than normally would have.
The authors then used this model to predict how many additional deaths would have occurred during the year starting on December 8th 2020—the day of the first jab—if no vaccines had been given, but all other factors remained the same. Excluding countries with tiny populations and China, where our excess-mortality figures are highly uncertain, the answer was 19.1m- 20.4m, 170% more than our estimate of the actual death toll during this period.
The study also found that vaccines helped rich and poor alike. Rich countries gobbled up 2.5 times as many doses per person as did the poor ones covered by covaxa jab-distribution scheme, but the paper found that nearly as many deaths were prevented in the covax countries (around 7m) as in rich ones (roughly 8m). This is proportional to the number of people in these groups of countries aged at least 65, who are most at risk from covid.
However, these calculations rely on a tenuous foundation. Because the authors could not obtain breakdowns of vaccine recipients by age group for the entire world, they assumed that all countries vaccinated their oldest residents first. In Africa, where just 3.5% of people are aged at least 65, this implies that a mere 5% vaccination rate would be enough to protect the bulk of the continent’s most vulnerable people. Any sizeable deviation from an age-based distribution would cause their estimate of lives saved in poor countries to fall, and make the distribution of jabs look less fair.
The paper also assumed that people’s behavior and the evolution of viral variants would have been identical in a vaccine-less world. In fact, without the jabs, individuals might have taken more precautions, and governments would probably have imposed longer lockdowns.
Yet as China’s struggle to sustain a zero-covid policy shows, public tolerance of lockdowns has limits, and new variants of sars–vsohv-2 are so contagious that no intervention can keep them at bay. As bad as the past 18 months have been, they would have been worse without vaccines—perhaps about 2.7 times worse.■
Source: “Global impact of the first year of covid-19 vaccination”, by OJ Watson et al., The Lancet Infectious Diseases, 2022