A vaccine that immunizes a person may not prevent transmission. For example, the virus would just keep spreading and reach people who haven’t been vaccinated.
Travel initiatives like vaccine passports wouldn’t stop people from picking up the virus overseas and bringing it home with new variants.
With a fast-moving pandemic, no one is safe, unless everyone is safe.
A vaccine can block transmission either by preventing people becoming infected or by stopping them passing the virus on even if they are infected.
One problem is that it’s very hard to measure transmission blocking directly.
A report published recently in March this year, from the European Centre for Disease Prevention and Control (ECDC), is the closest we have got. This looked at infections in household members of 150,000 healthcare workers in Scotland. However, because household members may have been infected by people other than the healthcare worker, and the people in the study hadn’t received their second dose, this study probably significantly underestimates transmission blocking.
What can be measured more easily is infection blocking. Regularly testing people to see if they are infected even if they have no symptoms.
A report published earlier in April this year on-line in CDC (Centres for Disease Control and Prevention) documented the Pfizer/BioNTech and Moderna mRNA vaccines as being 80% effective at preventing infection two weeks after one dose and 90% effective two weeks after two doses.
It is also possible that transmission blocking is even higher. It does appear as though people produce less virus if they are infected after vaccination and therefore, less likely to infect others. Indeed, one study has shown that people who still get infected after one dose of the Pfizer/BioNTech vaccine produce less virus on average.
Currently Australia is not using any of these particular vaccines in their mainstream program with the public. There is no data on the other vaccines regarding transmission.
Another important consideration is there are no plans to vaccinate children under 16 anytime soon. In countries such as the US, Australia and the UK that represents at least 20% of each country’s population.
Along with a relatively high proportion of adults globally who say they will refuse vaccination; these are important obstacles to raising herd immunity above the threshold needed to halt transmission.
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