Brazzaville/Geneva – In an incredible scientific feat, COVID-19 vaccines have been developed within less than a year after the virus was first detected. Several vaccines that have become available have shown different levels of efficacy. So what is vaccine efficacy? Dr Lee Hampton, a paediatrician and medical epidemiologist with GAVI, the Vaccine Alliance, explains.

What is vaccine efficacy and why is it important?

Vaccine efficacy measures a vaccine’s protection against a disease/pathogen in a vaccine trial. If a vaccine has, for example, 70% efficacy, it means a person vaccinated in a clinical trial is around two-thirds less likely to develop the disease than someone in the trial who didn’t get the vaccine. When you give a vaccine outside a clinical trial, we then refer to vaccine ‘effectiveness’. Compared with efficacy, effectiveness takes into account all the complexities of the real world, outside of a controlled clinical trial setting. So far several COVID-19 vaccines seem to be highly effective in real-world settings. However, we still need to assess the full consequences for each vaccine’s effectiveness against newly identified variants that cause COVID-19.

Can I easily compare the efficacy of different COVID-19 vaccines?

People should be cautious about comparing results from these different vaccine trials because clinical trials are set up differently and measure different things. All clinical trials provide rigorous data, but it makes it much harder to do direct comparisons between vaccines. For this you’d need a head-to-head trial, with the same protocol for all the vaccines, delivered and tracked in the same way.

With COVID-19 vaccines, different clinical trials have looked at how well vaccines protect against symptomatic and asymptomatic infection as well as how well they protect against severe disease, hospitalization and death. While it’s generally thought that protection from a given dose of vaccine fully kicks in about two weeks after that dose is given, some trials are designed to assess how much protection the vaccine provides starting immediately after that dose. Others will assess efficacy after two weeks following vaccination.

Another important difference is the way trials might measure outcomes.  To detect COVID-19 infection, for example, participants might just have to self report symptoms, or they might be given regularly scheduled blood tests for signs of the virus. Studies that report lower efficacy against infection might be more rigorous in trying to identify if someone was infected or not.

Is there a standard vaccine efficacy threshold and should a lower level be a concern?

There is no single standard vaccine efficacy threshold that can be applied to all vaccines. Any decision to use a vaccine or drug always involves weighing the benefits against the risk. In some vaccines, like the malaria vaccine, we are willing to accept relatively low efficacy because the problem is so severe, and the need for some protection is so great.

The 50% efficacy threshold set for COVID-19 vaccines is because COVID-19 was deemed such a severe disease, that if a vaccine is only 50% effective, it’s still worth using. Fortunately, the emerging data on COVID-19 vaccines suggests that the vaccines are very safe with high efficacy, at least against some of the variants.

The COVID-19 vaccines all have different efficacies. Which one should I take?

There has been a lot of focus on using COVID-19 vaccines to protect against symptomatic disease. But what we really care about how these vaccines protect against severe disease, hospitalization and deaths. The data so far shows that all these vaccines are better at protecting against severe disease, hospitalization and death than against symptomatic disease. This is because preventing a virus from getting to the point of severe disease is easier than preventing all symptoms.

For example, the AstraZeneca vaccine seems to have lower efficacy than the Pfizer vaccine at preventing mild to moderate disease, but has proven very useful in preventing severe disease, with nearly 100% efficacy against at least some variants of the virus that cause COVID-19. Given the very real risks from COVID-19, the high degree of safety we’ve seen so far from COVID-19 vaccines, and the evidence that COVID-19 vaccines provide protection, especially against severe disease, hospitalization, and death, I would personally take any WHO-recommended COVID-19 vaccine offered to me, and I have advised my family, including my parents who are in their late 70s, to do the same.