The Novavax vaccine had 100% efficacy against the original Covid strain in its US phase 3 trial (the one that originated in China), and it had 96.4% efficacy against the original strain in its UK phase trial. By comparison, Moderna had 94.1% efficacy against the original strain and Pfizer had 95% efficacy. So Novavax beat both Moderna and Pfizer against the original strain by between 1.4% and 5.9%.

At the time that the Moderna and Pfizer trials were completed, there were no variants – only the original strain. In both Novavax trials, a sizable percentage of cases were variants, against which all vaccines are at least somewhat less effective. This is because the vaccines were designed to beat the original virus, and the variants have mutated and evolved to become different from the original virus. So it’s not surprising that all the vaccines are at least somewhat less effective against the variants. 

Therefore, when an article notes that the Novavax vaccine had a combined efficacy of about 90% in the US trial and the UK trial, it doesn’t mean that it was less effective than the Moderna or Pfizer vaccines. It means that at least half of the cases were from variants that are somewhat better at resisting the vaccines. The UK number is a blend of 96.4% efficacy against the original virus, and 86.3% efficacy against the Beta variant.

In the US trial, it had 100% efficacy against the original virus and 93.2% efficacy against variants of interest and variants of concern (including against the Beta variant that was predominant in the UK trial). So in the US trial, Novavax’s numbers against the original virus and against the variants of interest and concern improved 4% and 6.7% respectively. 

The UK results were already the highest efficacy, so it was impressive that the US results were even stronger. In the US trial, there were a small number of cases with unknown mutations that were not the original virus nor variants of interest or concern. These brought the average somewhat lower than the 100% efficacy against the original and 93.2% against known variants. 

Since Moderna and Pfizer fared about 1 1/2% to 6% worse than Novavax against the original virus, they will likely perform similarly worse than Novavax against most variants. A study in Israel found that the Pfizer vaccine had only 64% efficacy against the Delta variant. This means that if half of a population has the original virus and half has the Delta variant, Pfizer’s combined efficacy would be about 79.5%. That is the average of 95% efficacy against the original and 64% against Delta. 

Thus, when comparing results, it’s essential to look at how each vaccine did against a particular strain.

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