More than half of the adults in Britain have now received at least one dose of Covid vaccine, but threats to vaccine supply, soaring case numbers on the continent and the emergence of new variants that are resistant to vaccines mean the country is in a precarious position.

How much protection does a single dose of a Covid vaccine give, and when?

It normally takes the body two to three weeks to muster a full immune response to a shot of vaccine, regardless of which one it is. The level of protection the shot affords varies depending on the particular vaccine, the health and genetics of the person who receives it, and how long after the shot they are exposed to the virus. The Joint Committee on Vaccination and Immunisation, which recommended delaying booster shots for three months in Britain, estimates that after three weeks the first Pfizer shot has an efficacy of about 90% and the first AstraZeneca shot has an efficacy of about 70%. This means that whatever your risk of getting symptomatic Covid beforehand, it is roughly 90% or 70% lower, depending on which shot you have. But these numbers must be treated with caution. The vaccine efficacies quoted here are called “point estimates” and there can be large uncertainties around them. Protection after one shot may be substantially lower in older people. Another uncertainty is how soon protection starts to wane.

What about two doses?

The second shot is intended to boost the immune response and make protection more long-lasting. Recent data from AstraZeneca’s US trial found that two shots of its vaccine achieved an efficacy of 76%, but the doses were given closer together than in Britain. A recent Lancet study showed that when the booster was delayed by 12 weeks or more, efficacy rose to 81%, compared with 55% when the shots were given less than six weeks apart. The Pfizer booster pushes efficacy up to 95% when given three weeks after the first shot, but the impact of delaying the booster is unclear.

Could I still catch Covid and become ill or even die after being immunised?

Yes, particularly if you are in a group that doesn’t respond well to the vaccine, says Stephen Griffin, an associate professor in the school of medicine at Leeds University. The vaccines reduce the risk of Covid but they don’t eliminate it. People who have been vaccinated can still get the virus and fall ill with the disease. The vaccines are very good at preventing hospitalisations and deaths – up to 100% in some clinical trials – but they will not work in everyone. The risk of infection, illness and death after vaccination could be far higher if new variants that evade vaccine-induced immunity become widespread in Britain.

Can I pass the virus on to other people?

It’s certainly possible. To transmit the virus, a person first has to become infected, and second produce enough virus in their upper respiratory tract to pass it on. Manufacturers and scientists are still investigating how well Covid vaccines prevent transmission of the virus. The early signs are good. Researchers at Oxford University suspect that a first shot of their vaccine reduces transmission by about two-thirds. Meanwhile, doctors at Addenbrooke’s hospital in Cambridge found that a single shot of the Pfizer vaccine could reduce asymptomatic infections by three-quarters. The emerging picture is that vaccines will reduce transmission, but not by as much as they prevent illness.

How likely are people to catch Covid and get sick or die, given that half of UK adults have now had at least one dose?

The risk falls with every shot of vaccine given, but the virus will still be in circulation. There are millions of people who have not been vaccinated, and others who will not have had a good immune response to the jab, and these people remain vulnerable to the virus. If Britain’s vaccine rollout is interrupted but the reopening of society continues according to dates rather than data, then we can expect a larger third wave, including a rise in deaths, in the summer or autumn. “Half the adults isn’t enough to attain population immunity, even with two doses,” says Griffin.

The risk to individuals depends on a host of factors: their health, how much virus is around, how much contact they have with others, and whether they and their contacts have been vaccinated. So far Britain has achieved enviable vaccine coverage, with more than 90% of people over the age of 70 or deemed clinically extremely vulnerable having had at least one dose. If all those people receive their second shots, and vaccination levels stay high in younger people, hospitalisations and deaths will fall dramatically, even if the virus is here to stay.

This content first appear on the guardian

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