Chile is being looked to as a bellwether for Covid-19 control as cases drop dramatically in Latin America’s most vaccinated county, but concerns about the impact of the Delta variant loom.

Over 60 per cent of Chileans are now fully vaccinated, a higher proportion of the population than the United States, the majority with Sinovac’s CoronaVac.

Daily infections have fallen from more than 8,900 new cases on June 6 to less than 1,500 in recent days, according to World Health Organization data, a significant shift from recent surges even as an vaccine programme was rolled out. The number of deaths has also fallen.

“The most intuitive explanation for this quick decrease in the number of cases is that it is the result of a large share of the population having received their full vaccination schedule,” said Eduardo Undurraga, an assistant professor at Chile’s Pontificia Universidad Católica who researches vaccine effectiveness.

But Undurraga said vaccination alone did not explain the decrease and pointed to other measures that remained in place, including mandatory masks, a ban on large gatherings and nighttime curfews.

Meanwhile, Chile was not at a point where it could relax, he said, especially as the highly transmissible Delta variant spreads globally, causing spikes in other highly vaccinated countries.

“The Delta variant could rapidly change the epidemiological situation in Chile, as has happened in Europe. We need to vaccinate as many people as possible and keep low-cost preventive measures, [such as] masks, ventilation, small gatherings, physical distance, in place to avoid an increase in transmission,” he said.

“Cases are now coming down, but countries in the region are looking at Chile’s example and calculating how best to balance vaccine distribution, social distancing and masking mandates to protect their pandemic-weary populations,” said Katherine Bliss, a senior fellow at the Global Health Policy Centre at the Centre for Strategic and International Studies’in Washington.

Bliss also noted that when Chile loosened certain control measures, such as restrictions on domestic and international travel earlier this year, it experienced a surge in Covid-19 cases despite already being well into its vaccination campaign.

Some 45 per cent of the national population of more than 18 million had been vaccinated in early June when cases spiked and the capital Santiago went into lockdown.

The last surge was driven by the unvaccinated, according to Undurraga. But it also raised questions about the effectiveness of CoronaVac, which is widely used in Chile and in lower and middle income countries around the world.

In a large-scale real world study, Chilean researchers found that the vaccine was 65.9 per cent effective against infection and 87.5 per cent and 86.3 per cent effective at preventing hospitalisation and death respectively. The figures fall below those for doses by Pfizer/BioNTech and Moderna, which passed 90 per cent in some real-world studies.

Chile has had limited access to those vaccines and relied on early deals cut with Beijing-based Sinovac for its campaign.

Over 18 million doses of CoronaVac have been administered in Chile, with around 5 million from Pfizer/BioNTech, and 400,000 from AstraZeneca – all two-dose vaccines. Half a million of the one-shot CanSino have also been given, according to government data.

Undurraga said vaccine effectiveness would be a factor in reaching herd immunity – a situation reached when there is enough immunity across a population to stem viral spread – but this may not be feasible through vaccination alone in the short term.

“Lower vaccine effectiveness, [for example] CoronaVac compared to Pfizer or Moderna, means you need a higher proportion of the population vaccinated to reach herd immunity,” he said.

“More transmissible variants also raise this threshold, and there are already confirmed infections with the Delta variant in Chile.”

Another variant, Gamma, is predominant in the country.

However, the spread of Delta, which the WHO expects to become dominant globally, has prompted a reassessment of vaccine effectiveness.

Cases among health care workers vaccinated with CoronaVac in Indonesia and Thailand have driven concerns about how well the jab holds up against the Delta variant.

There is currently no real-world data on CoronaVac’s effectiveness against Delta, though studies have shown other vaccines have reduced effectiveness against the strain.

A Chilean laboratory study of CoronaVac found neutralising antibodies reduced at least fourfold against the Delta variant compared with the original strain, according to an announcement of the study released this month.

Scientists involved, who also led the country’s late-stage clinical trial of CoronaVac, recommended a third dose of the jab, which could boost antibody response, according to Reuters.

The WHO has said that higher levels of antibodies are needed to overcome the Delta variant, but also maintains that blood antibody tests are not a clear indicator of how well a vaccine will work in the real world.

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