We want to get everybody vaccinated. Opening up to younger age groups could be one way to speed things up — but it will come with logistical challenges too.
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If you take evolutionary biology into account. Nowhere should have had phased rollouts where the virus is present in any significant number. They also shouldn't be vaccinating until they have enough doses stored to rapidly vaccinate everyone in the shortest period possible.
Younger people are more likely to be asymptomatic and therefore less likely to spread the virus than older people. So they vaccinate the older people first. When the virus is in the community spreading. This is putting an evolutionary pressure on the virus to adapt to be more symptomatic and able to spread amongst younger people. Putting more people at risk.
When you're slowly rolling out the vaccine, you're giving the virus plenty of time to adapt to new variants to get around the vaccine. As opposed to vaccinating everyone extremely rapidly and the virus is stopped without time to evolve.
I suspect some of the new more dangerous variants evolved in places where there had already been a lot of spread of the virus. As a result of ever-growing sections of the population having some immunity. This created a pressure on the virus to become more contagious and dangerous. This is what slowly vaccinating people in a place with the virus spreading will do.
Just look at this article from February.
Survey finds 56% of Delhi has COVID antibodies, adding to India’s declining cases mystery
Covid cases declining, more than half the population has antibodies. All across India anti-body testing was showing the real numbers of cases was extremely higher than the official tally. Which is why I harp on about comparing case numbers and deaths between a lot of countries isn't at all accurate. Anyway, lots of people with anti-bodies, cases dropping and then BOOM. A new dangerous variant evolves that is worse.
A slow vaccine rollout creates similar evolutionary pressures. It's one of those curious areas of "we follow and trust the science" that has been ignored. It appears no consideration or at least public discussion has been made about this by those in charge of the vaccine rollouts across the globe.