When we think of the Pfizer (NYSE:PFE) coronavirus vaccine, we think of strong performance. The company wowed the global health community when it reported overall efficacy of more than 90% in six-month follow up of its phase 3 clinical trial.

Pfizer has fully vaccinated more than 86 million Americans and is the leading vaccine provider in other countries too -- such as Israel. And speaking of Israel, the country announced news last week that paints another picture of the Pfizer vaccine's effectiveness. This may seem like bad news on the surface. But it might not be all that negative. And it actually may lead to more revenue for Pfizer. Let's take a closer look.

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A drop in coronavirus prevention

First, the news. The Pfizer vaccine's efficacy in overall coronavirus prevention declined to 39% during the one-month period ending July 17, according to Israel's Health Ministry. That's down from 64% in the period of June 6 through the early days of July. Effectiveness in severe disease is decreasing at a much slower rate. It's down to 91% from 93% in the earlier report. At the same time, the highly contagious delta variant is gaining ground.

Here's why investors may initially worry: The numbers alone seem to say the vaccine isn't working well against today's coronavirus variants -- primarily delta. And that's enough reason for governments to think twice before ordering more Pfizer doses.

But here's why investors shouldn't worry. First, efficacy in preventing severe coronavirus remains extremely high. It would be great to prevent every coronavirus infection. But what's most important is preventing the worst infections and death. The Pfizer vaccine continues to do just that.

Next, we have to look beyond the numbers. Israel launched a rapid and aggressive vaccine rollout early in the year. Many older people and those with comorbidities received vaccination in January. Six months down the road, it's likely immunity is waning.

Vaccine efficacy or declining immunity?

So, the issue we're facing may not be one of vaccine efficacy against the delta variant. Instead, it may simply be the issue of declining immunity. And this shouldn't be a surprise. Pfizer CEO Albert Bourla told CNBC a few months ago that a booster shot probably would be needed six to 12 months after initial vaccination. He even mentioned that variants may be a determining factor.

This matches the situation in Israel. Efficacy looked better there a few months ago when vaccination was more recent. Now, immunity has decreased over time, and variants have gotten tougher. So, the need for a booster is increasing.

And this isn't a detail unique to Pfizer. Rival vaccine maker Moderna also has spoken of the necessity of a booster in about the same timeframe.

This brings me to how all of this could equal higher revenue for Pfizer. The situation in Israel looks like real world evidence that a booster is needed now or soon -- at least for people who were vaccinated earlier in the year. And that would apply to vaccinated individuals anywhere in the world.

Pfizer said this month that it plans on seeking regulatory authorization for a third dose of its vaccine to be given as a booster. Health leaders weren't immediately supportive of the idea. The U.S. Food and Drug Administration and the Centers for Disease Control and Prevention said around the same time as the Pfizer announcement that anyone who is fully vaccinated doesn't need a booster right now. But with cases quickly rising, that message could change sooner rather than later.

An extra $1.7 billion

If it does, Pfizer doses may fly off the shelves. If every American fully vaccinated with Pfizer so far opts for a booster, we're looking at more than 86 million vaccine doses. At the price the U.S. originally paid for Pfizer vaccines, that represents $1.7 billion.

Of course, this doesn't mean the U.S. will order an extra $1.7 billion in doses right away. But as the current supply decreases, the country may be quick to replenish -- and stock up for the next round of vaccinations. We may see this happen not only in the U.S., but worldwide.

This movement will take some time. Any increase in orders and revenue may happen next year and into the future. But right now, it looks like the stage is being set for that outcome. That's good news for those seeking maximum protection from COVID-19 -- and good news for Pfizer and its investors.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.