As the United States moves toward covid-19 herd immunity, one thing is particularly frustrating: We don’t know what herd immunity will actually look like.

This is not an indication of failed scientific inquiry. Rather, it results from hard limits on our knowledge of a new virus. We know from long experience, for example, that the measles virus is wickedly contagious. One person with measles will infect up to 90 percent of the non-immune people they encounter. To prevent outbreaks, it takes about 90 percent vaccine coverage.

With the coronavirus, scientists don’t have enough information to make such definitive predictions. As of now, epidemiologists at the National Institutes of Health estimate that covid-19 will require 70 to 85 percent immunity for the human herd to be safe. That estimate’s relatively wide range results from scientific uncertainty. How much more contagious are new variants of the virus? Will variants begin to defeat existing vaccines? (So far, the vaccines seem to hold up well against new covid-19 variants, but that could change.) How long does natural immunity last?

In a recent conversation, Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases, discounted the value of detailed estimates about herd immunity. “Instead of fixating on a mystical number,” he told me, “we can say with certainty that the more people who are vaccinated, the less infections you will get.” Fauci pointed to Israel’s high vaccination rate as an example. “You reach a crossroads when the level of vaccination is so high that disease plummets.”

Fauci insists that the United States’ current seven-day average of about 40,000 new covid-19 cases a day remains too high. “We can’t keep it there,” he said, even if “some people say that a low level is not a big deal.” The sick and weak remain at serious risk. “Until we have better therapies,” he said, “they are still highly vulnerable to dying.”

So we can’t be content with periodic covid-19 surges. Fauci believes the target embraced by President Biden of 70 percent vaccine coverage by July 4 is realistic and would result in a “significant diminution” of the disease. “If we get to 85 or 90 percent" coverage, he added, “we can stuff this virus.”

The United States has been a source of lifesaving innovation on covid-19, and vaccine supply is no longer our most urgent problem — hesitancy is. Even so, we can and must begin to think creatively about the humanitarian crisis of acute vaccine shortages globally. With nations including India, Brazil and Colombia on the brink of disaster, we have a moral duty to bring our innovations to vulnerable and desperate people.

This is a challenge analogous to the one the George W. Bush administration faced in the early 2000s. After the AIDS crisis left its horrible trail of tragedy across the country, the United States was largely meeting its own demand for miraculous new AIDS drugs. But Africans were dying in endless waves because they lacked them. The President’s Emergency Plan for AIDS Relief (PEPFAR) was designed, in part, to bring antiretroviral medicine to places much of the world had given up on. Despite large logistical challenges, it worked.

In our current crisis, the Biden administration has proposed to waive patent protections on vaccines to encourage local production in affected parts of the world. The problem with this approach is not primarily ideological; it is practical. At least for the next year, the ability of new manufacturing facilities to make a difference in the worldwide supply of vaccine doses will be limited.

To meet an emergency need, the first obvious step is to share surplus doses that are building up as we wait on the vaccine-hesitant. But it will also be necessary for existing vaccine producers to multiply their production. So why not just create a PEPFAR-like program that pays a few billion dollars for manufacturers to produce a few billion new doses that would go to coronavirus disaster areas?

The problem is limitations on production. To substantially increase output, Pfizer, Moderna and Johnson & Johnson would need raw materials for the supply chain, highly sophisticated new equipment and training for new staff. While an infusion of money might help with equipment and personnel, the limits on raw materials are harder to overcome. Paradoxically, if the waiver of intellectual property protections for vaccines inspires less-efficient, lower-quality production efforts, these will draw on the same limited raw materials, which could reduce the overall global availability of vaccine doses in the short term.

Regardless of these obstacles, it seems clear that additional production from existing sources will scale up faster than the sharing of intellectual property and the building of new facilities. In this case, the Biden administration should be doing everything it can to help proven manufacturers expand. Even if that requires a PEPFAR-like effort for our time.

Read more: