Malta is basing its herd immunity calculations on the share of people who have received just one COVID-19 vaccine dose, public health chief Charmaine Gauci revealed on Friday.

Around 57 per cent of over 16s have received at least one vaccine dose so far, based on Health Ministry data and the National Statistics Office's most recent population figures. 

That figure falls to just under 49 per cent when all Maltese residents, including children, are factored in. 

Authorities are administering four different vaccines approved by European regulators. Three of those are double-dose vaccines. A fourth, made by Johnson & Johnson, is a single-dose inoculation and only started being administered locally this week. 

Health authorities have previously said that they aim to achieve “herd immunity” by the end of June, when they aimed to have vaccinated 70 per cent of adults.

Until Friday, they had not explained whether that 70 per cent target referred to the proportion of people inoculated with a single vaccine dose, or those fully vaccinated with two doses.

The European Union has set member states a target of vaccinating 70 per cent of adults by the end of summer 2021, but has not defined “vaccinated”.

Speaking on Friday, Gauci said that the herd immunity target would be based on those given a single vaccine dose because studies had shown that even a single dose caused a strong immune response in patients.

"We will have a substantial number of people who would already have received their second dose," Gauci added. 

More than 115,000 people were fully vaccinated against COVID-19 as of Thursday. 

But while Malta is basing its vaccine targets on the number of people given one dose, it is setting a higher bar for vaccine certificates: only people who are fully vaccinated with two doses will be eligible for a certificate, Gauci indicated. 

What does the science say?

Gauci's claim about the robust immune response provoked by a single dose appears to be backed up by the scientific literature.  

Late-stage trial data published by the Lancet in February found that a single dose of an AstraZeneca vaccine was 76 per cent effective at stopping COVID-19 infections. A study in Israel provided similar results for those given a single shot of the Pfizer COVID-19 vaccine, finding that it was up to 85 per cent effective at preventing infection. 

Studies into the vaccines’ effectiveness at preventing hospitalisation, rather than infection, are just as encouraging: a research paper published last month found that a single shot of a Pfizer vaccine was 91 per cent effective at preventing hospitalisation at 28 to 34 days from vaccination. The AstraZeneca vaccine was 88 per cent effective.  

Those findings were bolstered by the results of a real-world study in South Korea, published this week. It found that a single dose of a Pfizer vaccine was more than 89 per cent effective at preventing COVID-19 infection. A single dose of an AstraZeneca vaccine was almost as effective, at 86 per cent.  

Studies into the Moderna vaccine suggest similar results.

What could go wrong?

Despite the encouraging numbers, immunologists continue to recommend that people take both vaccine doses to ensure the highest level of protection.

Clinical data published in Science, for instance, suggests that a single dose of the Pfizer vaccine is not enough to provide protection against a UK virus variant in people who have never had COVID-19 before. 

The majority of current COVID-19 cases in Malta are of that UK variant. 

Receiving a second dose is also important to ramp up antibody production to their maximum levels: experts still do not know how long vaccine protection will last. 

Can ‘herd immunity’ be achieved?

While Gauci, Health Minister Chris Fearne and even the European Commission have equated herd immunity to their target of vaccinating 70 per cent of over 16s, the truth is that nobody really knows when – or if – it will kick in for COVID-19.

Immunologists are increasingly warning that the rising number of virus variants, coupled with evidence that virus immunity wanes following infection, suggest that herd immunity will be impossible to achieve for COVID-19.

They instead warn that the virus will continue to circulate and could require annual booster shots to keep in check, much like the flu.

Even if the virus behaves as predicted, people's unpredictable behaviour could yet make herd immunity harder to achieve, by dropping their guard after getting jabbed. 

Those who have received a vaccine dose are more likely to increase their social activity, increasing the risk of further virus spread and shifting the herd immunity goal posts. 

The vaccine is not bulletproof,” Dvir Aran, a biomedical data scientist at the Technion — Israel Institute of Technology in Haifa, told Nature.

“If before the vaccine you met at most one person, and now with vaccines you meet ten people, you’re back to square one.”