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It doesn't make sense to try to put a percentage figure on the achievement of herd immunity to covid-19 ( 23 January, p 12 ). Going to the extreme for illustration, in a sparsely populated country where you rarely interact with other people, your chance of getting covid-19 is slim, even if the incidence is …

Rather than the industrialised West supplying vaccines to low-income countries, we should spend some money helping them build the facilities to self-supply, which would produce long-lasting economic and employment benefits at the same time ( 6 February, p 21 ).

The article "How to give your vaccine a boost" mentions hugging, which is tricky right now ( 13 February, p 8 ). It seems likely that hugging a tree would have similar benefits.

Your look at the pros and cons of using hydrogen as a fuel was great, but it failed to delve into the positive impact on the UK's balance of payments ( 6 February, p 44 ). Investing billions into truly clean hydrogen could save trillions by reducing imports of oil and gas.

Roger Elwell writes that, as a non-city dweller, he would be unhappy to see his taxes pay for free travel in the likes of London ( Letters, 13 February ). He appears to be perfectly happy, however, to take advantage of cost-inefficient rural highways and services, all – inevitably – subsidised by those who live …

The problem of carp in Australian rivers has one simple solution: eat them ( 13 February, p 20 ). Baked carp is an excellent dish. Your correspondent Sam Wong could provide a recipe. Considered alongside the article on the fishing industry in the same issue ( p 36 ), it seems cavalier to throw away …

Thank you for your comprehensive article "A rescue plan for nature" 20 February, p 34 . The focus was biodiversity loss, but it could equally have been on climate change or pollution. These are all symptoms of the problem of our consumption, compounded by a growing population. While the focus remains on the symptoms and …

Despite calls to put global interests above national interests when it comes to coronavirus vaccines, we should wave a flag for the UK's approach ( 6 February, p 12 ). While it is well ahead in terms of vaccine supply compared with many countries thanks to its bilateral deals, it is giving £548 million to …

Craig Hutton writes that to build a Dyson sphere, a civilisation would have to raid a vast number of star systems for materials ( Letters, 20 February ). However, according to my rudimentary maths, just one small planet may do the job. Mercury is almost entirely made of metal. If you flattened it into a …

Following up on readers' tips for beating pesky flies Letters, 13 February . If you swat a fly simultaneously from two different directions, its on-board computer experiences data overload and the fly remains fixed to the spot.