Covid-19: What’s the Best Vaccine?

As scientists raced to develop Covid-19 vaccines, public health specialists were hoping that more than one group would succeed. Having multiple companies producing vaccines would make it easier to inoculate a lot of people fast. Be careful what you wish for. A range of vaccines with different efficacy results now has given rise to worries that some people may refuse the shot on offer in hopes of getting a “better” one later. In reality, comparing efficacy numbers isn’t necessarily the best way to measure a vaccine’s value. And as suppliers struggle to meet global demand, experts say the best vaccine for you is probably whichever one you can get now.

What does efficacy mean?

On a basic level, vaccine efficacy of 50%, for example, roughly means that an immunized person has a 50% reduced risk of becoming ill compared with an otherwise similar non-immunized person. However, the measurement can be applied to different questions about a vaccine’s effect. For example, several Covid-19 vaccines appear to successfully -- 100% -- avert hospitalization and death. But since relatively few people infected with SARS-CoV-2 become critically ill, it’s hard to measure such a rare outcome reliably in clinical trials involving only tens of thousands of participants -- a relatively small pool. Instead, the primary aim of most late-stage trials has been to measure broader efficacy against lab-confirmed Covid cases with any symptoms, including mild ones.

What efficacies are being reported?

The first two Western vaccines to prove effective -- one from Pfizer Inc. and BioNTech SE, and another from Moderna Inc. -- set a high bar, with efficacy estimated at 95% and 94%, respectively. That means that Covid cases among trial participants who received the vaccine were reduced by that much compared with those who got a placebo. Efficacy was estimated at 66.7% for AstraZeneca Plc’s and 89.3% for Novavax Inc.’s two-shot regimens; and 66.9% for Johnson & Johnson’s single-shot vaccine.

Are the numbers reliable?

It’s hard to say. Data from the clinical trials have been reported in various ways and subject to varying degrees of scientific scrutiny. Although publication in a peer-reviewed, scientific journal is considered the gold standard for ensuring the accuracy, integrity and credibility of clinical data, only a handful of Covid vaccine studies have undergone that rigorous vetting process so far. Vaccine efficacy data from other studies have been reported in press releases, articles in state-owned media and in papers released on so-called pre-print servers and, therefore, weren’t reviewed by scientists not involved in the research.

Why isn’t efficacy all that counts?

For one thing, the figures aren’t directly comparable. That’s in part because the vaccines weren’t tested using the same criteria or groups of people. Also:

The vaccines were tested at different times and in different places. The intensity of the epidemic and measures to mitigate it, such as mask-wearing, may contribute to differences in efficacy estimates between countries.

SARS-CoV-2 has mutated over time, generating variants that appear to be more dangerous. So, in general, the first vaccines to prove effective likely faced fewer of these viral strains than subsequent ones have.

Vaccines take time to work, and the time periods during which efficacy was measured in clinical trials differ across studies.

While most of the trials were designed to evaluate how well vaccines prevented any symptomatic case of Covid, the J&J vaccine was tested for its ability to protect against moderate and severe Covid, which entails having at least two lesser symptoms or one or more serious one, such as an elevated respiratory rate.

Public health officials say that, at least initially, the best vaccine is whichever one is available at the time of eligibility.

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