How good is the Oxford/AstraZeneca Covid vaccine?
Interim results from phase 3 trials in Britain and Brazil found the vaccine to be 70.4% protective. More than 20,000 volunteers were involved in the trials and half of those were in the UK. Investigators recorded 30 cases of Covid-19 in people who had two doses of the vaccine and 101 in those who received a control jab. In those who received two full doses, the vaccine was 62% protective. But it appears to work better when given initially as a half-dose followed by a full dose, with protection then rising to 90%.
Is it safe?
The safety analysis comes from a database of 24,000 volunteers who took part in the clinical trials in the UK, Brazil and South Africa, with follow-ups since April. No serious safety events related to the vaccine have been identified, the scientists say. That will be reassuring, since the trials were paused after one volunteer in the UK fell ill. Details have not yet been released of mild side-effects, such as a sore arm or fatigue, after the jab.
Does it protect everyone to the same level?
The elderly are most at risk from Covid, so scientists have been keen to see a good response in older trial volunteers. Many vaccines for other infections are less effective in the elderly because the immune system wanes with age, but data from Oxford suggest there has been a “similar” response in younger and older adults.
How does it work?
At the heart of the Oxford/AstraZeneca vaccine is a virus that causes the common cold in chimpanzees. The chimp virus is modified so it cannot multiply and cause disease in the body. It is then loaded up with the gene for the coronavirus spike protein, the club-shaped part that dots the surface of the virus and is used to penetrate human cells.
When the vaccine is injected, the chimp virus delivers the coronavirus gene to human cells which start to churn out the spike protein. These are detected by the immune system which produces antibodies and other responses that can attack the real coronavirus should the person become infected in the future.
How does it differ from other vaccines?
The Oxford vaccine, named ChAdOx1 nCoV-19, is based on a different technology to the two recent “mRNA” vaccines from Pfizer/BioNTech and Moderna, both of which performed well against Covid. No mRNA vaccine has ever been approved by regulators, but the Oxford approach has been used in vaccines given safely to thousands of people of all ages for diseases ranging from TB and malaria to Mers (another coronavirus) and Ebola, for which it is now being used in Uganda and the Democratic Republic of the Congo. Oxford opted for the chimp virus because it has the potential to generate a strong immune response and, since it cannot grow in humans, it should be safe.
Two other frontrunner vaccines, from BioNTech (developed with Pfizer) and Moderna (developed with the US National Institute of Allergy and Infectious Diseases), perform well, according to interim results from phase 3 human trials, achieving more than 90% efficacy. These are based on technology that is new to the vaccine field. Both use genetic material called mRNA, or messenger RNA, which tells human cells how to make coronavirus spike proteins. The vaccines use lipid nanoparticles to deliver the mRNA into muscle cells which manufacture the spike protein and prime the immune system, guarding against infection. In principle, mRNA vaccines may be faster to develop and easier to manufacture.
While BioNTech’s vaccine must be stored at -70C to -80C, making distribution a complex logistical challenge, Moderna’s vaccine can be stored in a fridge at 2C to 8C for a month. Oxford’s vaccine can also be stored in standard refrigerators, making distribution far easier. Other coronavirus vaccines are under development and use the virus itself in a weakened or inactive form. This is the same approach used in highly effective vaccines for polio and measles.
How much will it cost?
The Oxford vaccine is far cheaper than others that have shown efficacy. AstraZeneca aim to sell the Oxford vaccine for $3-4 per shot, compared with $20 for the Pfizer/BioNTech shot and $25 for Moderna’s.
How many doses will Oxford manufacture?
AstraZeneca has drawn up agreements to make 2bn doses of its vaccine by next summer. Through Europe’s inclusive vaccines alliance, up to 400m doses of the Oxford vaccine will be supplied to European nations, starting in 2020. The UK already has 4m doses of Oxford vaccine with 96m to come. Further orders include 40m doses of the Pfizer/BioNTech vaccine and 5m doses of the Moderna vaccine.
What questions remain?
One big question surrounding all of the coronavirus vaccines is whether they prevent asymptomatic infections as well as illness caused by the virus. Reducing asymptomatic infections, when people can spread the virus without knowing they have it, could have a big impact on slowing the spread of the disease. The Oxford data found some evidence of reduced asymptomatic infections in those who received the half-dose followed by a full dose later.
No one knows for how long the Oxford vaccine, or any other coronavirus vaccine for that matter, provides protection. Some vaccines are effective for years, but for obvious reasons this will take longer to confirm.
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