With Covid-19 shots in
1. How good are Covid vaccines?
In general, the Covid shots are
2. What are the caveats?
The evidence so far is skewed, with Israel, the U.S. and U.K. supplying the majority of the data, which pertains mostly to vaccines made by
3. Are some vaccines better than others?
4. Why is it hard to tell?
Efficacy levels estimated from clinical trials aren’t necessarily comparable because each vaccine study used different regimens and measurements. They also were undertaken in different groups of people and at different times, when different strains were prevalent. In some countries, like Chile, two vaccines have been administered
5. How durable is the protection?
We’ll know as more time goes by. An unpublished study by Pfizer researchers that followed vaccinated individuals for up to six months reported a gradually declining trend in vaccine efficacy against a symptomatic infection, but found their shot remained “highly efficacious” overall. Efficacy peaked at 96.2% in the two months after a second shot, and declined gradually to 83.7% from four months, and then by an average of 6% every two months. Waning vaccine-induced immunity is likely to be first detected by vaccine failures in vulnerable individuals, such as diabetes and heart-failure patients, and reflect lower levels of virus-blocking antibodies. U.K. researchers says that a likely scenario is that, as immunity gradually wanes, more vaccinated individuals will get infected, but their immune system will rapidly control the infection, resulting in no or mild symptoms. However, exposure to the virus will further boost and broaden their immune response to it. Vaccination campaigns against SARS-CoV-2 may continue for years, though follow-up research is needed to understand when
6. What effect are variants having on the potency of vaccines?
They differ. Delta, the strain first reported in India in October, is the most worrisome detected so far. Twice as transmissible as the coronavirus that sparked the pandemic, delta is becoming dominant globally. Studies from the U.K. have shown that it’s more resistant to vaccines than the alpha variant that emerged there in late 2020, particularly in people who have received just the first dose. Some researchers say that will require a higher proportion of people in the community to be immunized to slow, and eventually stop, transmission through herd immunity. The reduction in vaccine effectiveness caused by delta may vary by type. The Pfizer-BioNTech shot appeared to be more effective than AstraZeneca’s in a study published in the New England Journal of Medicine in July.
7. What makes delta more tricky?
Vaccines have been shown to reduce the concentration of virus particles, or viral loads, in the airways of individuals who become infected. But delta is associated with viral loads that are 1,200 times higher in newly infected individuals compared with the original coronavirus strain -- a feature that helps delta spread as
The Reference Shelf
- Related QuickTakes on coronavirus
variants, including delta, vaccine nationalism, Covid vaccine reactions, breakthrough infectionsand mRNA vaccines.
- A list of Covid vaccines evaluated by the WHO, vaccine effectiveness studies, and the agency’s vaccine toolkit.
- The University of Melbourne’s weekly Covid vaccine updates.
Natsuko Imaiand Neil M. Ferguson interpret the estimates of Covid-19 vaccine efficacy and effectiveness.
- European epidemiologists assess vaccine effectiveness in seniors.
(Adds additional research in sections 5 and 7.)
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