Although the clinical trials are not complete and there are still many questions to be answered, it is difficult not to be optimistic about Pfizer’s new COVID-19 vaccine.

If there was a single physician or scientist who predicted a 90% protection rate, he or she did an amazing job of keeping it secret. If the vaccine’s unexpected preliminary results hold up, we will have the possibility of making a serious dent in this terrible pandemic, which has thrown the world into chaos in less than one year.

Vaccines have been remarkably successful at eradicating deadly diseases such as smallpox, polio, and diphtheria. Other vaccines, especially for influenza, are far from perfect. Flu vaccines must be given annually and have a protection rate of only about 50%. This is due to resistance and new strains of flu arising every year, rendering the previous year’s vaccine ineffective. This is already occurring with COVID-19 as numerous strains of the virus that causes it have already been identified.

It is all but certain that SARS-CoV-2, a strain of the coronavirus that causes COVID-19, will behave more like flu than polio. As a result, we will not know for years how long these vaccines will protect us against the infection. This is one of several reasons we believe the development of effective, direct-acting antiviral drugs to augment vaccines is essential. And as was the case with HIV/AIDS and hepatitis C, it is likely that a combination of drugs, or a drug cocktail, will be required.

At this time, we have nothing of the sort. Although Remdesivir recently became the first and only FDA-approved treatment for COVID-19, it is not potent enough to solve the problem by itself. Over the past nine months, a parade of “promising” repurposed drugs marched across the headlines. Some failed and some are still being evaluated, but there is no clear winner yet. This is not unexpected; repurposed drugs were originally designed for other conditions.

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There is another group of drugs being developed for which preliminary results are encouraging. Not surprisingly, this is due to these drugs utilizing time-tested mechanisms for inhibiting RNA viruses, including nucleoside and protease-based inhibitors, as well as others that stop the virus from replicating. We believe that drugs like this are essential for a number of reasons.

One is distribution: Worldwide distribution of the vaccine, especially to developing countries, will be extremely challenging based on the unprecedented number of people who must be vaccinated.

Two is cold storage: Most vaccines are unstable at room temperature and must be refrigerated. By contrast, small-molecule antiviral drugs (pills) have long shelf lives even at ambient temperatures. The Pfizer vaccine presents a special challenge since it must be kept at minus-78 degrees celsius, which is well below the range of normal freezers. This makes storage and distribution challenging.

Another reason we feel these drugs are essential is resistance: As mentioned above, numerous coronavirus strains are already circulating worldwide. It is unlikely a single vaccine will protect against all of them. Problematic is that the SARS-CoV-2 strain is a zoonotic virus, meaning it jumps from animals to humans, unlike polio or smallpox. This makes protection from SARS-CoV-2 much more difficult due to its inherent difference in normal vector-borne transmission.

A final reason is incompatibility: There are numerous cohorts of people who cannot be vaccinated due to underlying health conditions, a compromised immune system, or an allergy to the vaccine or one of its components.

In short, while there is certainly reason to be optimistic that this vaccine will be available soon and make a marked difference in our lives, there are still multiple reasons why we will need to continue to search for new, effective antiviral drugs to fight COVID-19 and SARS-CoV-2.

Jonathan “Josh” Bloom is a senior vice president and the director of chemical and pharmaceutical science for the American Council on Science and Health in New York City. Katherine Seley-Radtke is a professor of chemistry and biochemistry at the University of Maryland, Baltimore County. They wrote this for the News Tribune.