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Is there no scientific analysis of how many people the world can sustain for everyone everywhere to have a healthy, decent standard of living without overcrowding, while restoring biological diversity at the same time? With such information, we could talk about the best way to achieve this equitably ( 14 November, p 34 ). Attitudes …

Fewer people would be helpful, but there are two key caveats to this. Firstly, Paul Ehrlich went on to qualify his book The Population Bomb by noting that a well-off US family could have the same impact on resources as a Bangladeshi village. Secondly, deaths have to exceed births – we can all moan about …

Investing in education for girls is vital. It leads to women who have a bigger say in the number of children they have. For many high-income countries, pure self-interest is enough to aim for spending the recommended 0.7 per cent of GDP on foreign aid.

Referring to wealthier nations, you write about a "slow-burn issue of a growing 'dependency ratio': a large, ageing, economically inactive population supported by tax receipts from a dwindling band of working people". Presumably this is because we oldies, who do a lot of volunteer work, don't get paid for it, so are "economically inactive"? Who …

The importance of general education in reducing the overall birth rate is clear. However, an additional element of education is generally missed. If it can be agreed that a continually rising population isn't good for Earth as a whole, then a directed worldwide programme of education on this point should be implemented.

During my life, global population has tripled. This goes against the idea, raised in the leader related to your feature , that we know what works to limit population growth, which should perhaps carry the caveat "if it weren't for human nature". What chance is there that human nature will change, that we will overcome …

You point out that many nations will soon have falling populations. Let us rejoice in this. For the record – {06 December 2020} • In our population feature, the source of the cholera outbreak in Soho, London, in 1854 was the contaminated water from a communal pump, not the pump handle itself.

I am glad to read that the University of Oxford's Sarah Gilbert thinks more research is needed into the "half-dose" findings on the vaccine jointly developed with AstraZeneca ( 28 November, p 7 ). So far, nearly all commentary on the half-dose observation has been along the lines of coy smiles at a piece of …

The good news regarding the successful results of covid-19 vaccine trials is most welcome. However, as someone who is at high risk for this disease, I wonder if there will be any way of testing, post-vaccination, to find out if I am protected. I wouldn't feel confident to return to "normal" life unless I'm sure …

Reporting on the vast task of vaccinating people everywhere against coronavirus, Carrie Arnold quotes Saad Omer, director of the Yale Institute for Global Health, as saying: "There's no muscle memory to vaccinate people at that scale throughout the world, at levels that are needed to open up society ( 21 November, p 36 )." There …