More than half of UK adults have now received at least one dose of the Covid vaccine, but threats to the vaccine supply, the surge in the number of cases on the continent and the emergence of new vaccine-resistant variants mean that the country is in a precarious position.
How much protection does a single dose of a Covid vaccine provide and when?
It normally takes two to three weeks for the body to achieve a full immune response to any vaccine injection. The level of protection offered by the vaccine varies depending on the particular vaccine, the health and genetics of the person receiving it, and the length of time it is exposed to the virus after the vaccine. The Joint Committee on Vaccination and Immunization, which has recommended delaying booster injections for three months in Britain, estimates that after three weeks the first injection of Pfizer has about 90% effectiveness and the first injection of AstraZeneca has an efficiency of about 70%. This means that whatever your risk of getting symptomatic Covid beforehand, it is about 90% or 70% lower, depending on the vaccine you have. But these numbers should be treated with caution. The vaccine efficiencies cited here are called “point estimates” and there can be large uncertainties around them. Protection after a single blow can be significantly lower in the elderly. Another uncertainty is how quickly protection begins to decline.
What about two doses?
The second hit is meant to boost the immune response and make the protection longer lasting. Recent data from the US trial of AstraZeneca revealed that two injections of its vaccine achieved 76% effectiveness, but the doses were administered more closely than in Britain. A recent Lancet study showed that when the booster was delayed by 12 weeks or more, the effectiveness increased to 81%, compared to 55% when the injections were given less than six weeks apart. The Pfizer booster increases effectiveness by up to 95% when given three weeks after the first shot, but the impact of the delayed booster is unclear.
Can I still catch Covid and get sick or even die after being vaccinated?
Yes, especially if you are part of a group that is not responding well to the vaccine, says Stephen Griffin, associate professor at Leeds University School of Medicine. Vaccines reduce the risk of Covid but do not eliminate it. People who have been vaccinated can still catch the virus and get sick. Vaccines are very effective at preventing hospitalizations and deaths – up to 100% in some clinical trials – but not everyone will work. The risk of infection, disease and death after vaccination could be much higher if new variants that escape vaccine-induced immunity become widespread in Britain.
Can I pass the virus on to other people?
It is certainly possible. To transmit the virus, a person must first be infected and then produce enough virus in their upper respiratory tract to transmit it. Manufacturers and scientists are still studying how well Covid vaccines prevent transmission of the virus. The first signs are good. Researchers at the University of Oxford suspect that a first injection of their vaccine reduces transmission by about two-thirds. Meanwhile, doctors at Addenbrooke’s Hospital in Cambridge have found that a single injection of the Pfizer vaccine can reduce asymptomatic infections by three-quarters. The emerging picture is that vaccines will reduce transmission, but not as much as they prevent disease.
How likely is it that people will catch Covid and get sick or die, given that half of UK adults have now received at least one dose?
The risk decreases with each vaccine injection given, but the virus will still be circulating. There are millions of people who have not been vaccinated, and others who will not have had a good immune response to the bite, and these people remain vulnerable to the virus. If the vaccine rollout in Britain is halted but the reopening of the company continues based on dates rather than data, then a larger third wave, including an increase in deaths, can be expected in summer or fall. “Half of adults are not enough to achieve the immunity of the population, even with two doses,” Griffin says.
The risk to individuals depends on a multitude of factors: their health, the amount of virus around, the number of contacts they have with others and whether they and their contacts have been vaccinated. Britain has so far achieved enviable vaccination coverage, with more than 90% of people over the age of 70 or considered clinically extremely vulnerable having received at least one dose. If all of these people get their second injection and vaccination rates remain high among young people, hospitalizations and deaths will drop dramatically, even if the virus is here to stay.