The Economist explains
THE WORLD is spoilt for choice when it comes to covid-19 vaccines. More than 80 are in clinical development and 182 are in preclinical development. But people naturally want to know which works best. The reported numbers vary considerably. Pfizer and Moderna, two drugmakers that developed the same type of vaccine, have announced efficacy rates of 94-95%. On March 22nd AstraZeneca, a British-Swedish drugmaker, reported that its vaccine was 79% effective at preventing symptomatic covid-19 in trials in Chile, Peru and the United States. In fact, studies show that leading jabs work similarly well in the real world. So what do these numbers mean, and how is vaccine efficacy measured?
Vaccine efficacy is determined in randomised-control trials, in which volunteers receive either a vaccine or placebo. These trials tell researchers how well the vaccine works in optimal conditions, in which its storage and delivery are monitored and volunteers are healthy. To calculate efficacy rates, scientists compare the frequency of covid-19 infection in the vaccinated and the placebo groups.
Take the Pfizer jab, which reported efficacy of nearly 95% in November 2020. Trials were designed with the goal of reducing the severity of illness and symptoms, turning a severe case requiring hospitalisation into a mild cough that can be managed from home. But the efficacy rate refers to the number of confirmed infections. The company recruited around 43,000 volunteers, split evenly between a vaccine and a placebo group. Of the participants, 170 contracted covid-19, their cases confirmed by a positive nasal swab. And of these, 162 were in the placebo group and just eight were in the vaccinated group. That means only 5% as many vaccinated participants contracted the disease as did those in the placebo group. This yields the efficacy rate, meaning that under conditions similar to those of the trial, the Pfizer jab reduces the risk of catching covid-19 by 95%. It does not mean that 5% of participants who received the vaccine caught covid-19.
But not all vaccine trials are carried out in the same way. Some studies count people with mild symptoms as positive cases; others do not. Some have taken place in countries where partly immune-resistant variants of SARS-CoV-2, the virus that causes covid-19, are more common, or have tested only a single-dose regimen. Fortunately, direct comparisons between vaccines are now possible, based on millions of people who have already received different vaccines in the same country at the same time.
High efficacy rates are clearly desirable, but lower efficacy doesn’t mean a vaccine has failed. America’s Food and Drug Administration suggests that efficacy rates of 50% would be useful in the pandemic if the jab is widely deployed. The flu vaccine, for instance, has been shown to reduce risk of illness by 40-60% and saves thousands of lives a year. Moreover, efficacy in a lab differs from another measure: effectiveness, which looks at vaccine protection in messy real-life settings and on a larger pool of people. One paper, a preprint for the Lancet published on March 3rd, found that a single dose of either the Pfizer or AstraZeneca jabs cuts the risk of hospitalisation among people over 80 years old by 80%, starting 14 days after vaccination. Another study, in Scotland, included younger people and also found the two jabs had similar potency against hospitalisation. Trials are important for establishing the safety of a vaccine and how well it works. But those with differing efficacy rates can be similarly useful in the real world.
All our stories relating to the pandemic and the vaccines can be found on our coronavirus hub. You can also listen to The Jab, our new podcast on the race between injections and infections, and find trackers showing the global roll-out of vaccines, excess deaths by country and the virus’s spread across Europe and America.