Excitement about vaccines

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Why We’re Excited About Pfizer and Moderna Vaccine News

Vaccine efficacy is a measure of how well a vaccine works when vaccination goes perfectly according to plan. It’s calculated by looking at the risk of infection in vaccinated people compared to risk in unvaccinated people. Efficacy of 90% means that infection rates with the vaccine are 90% lower than seen without vaccination.

By Gregory Katz, MD, and Harry S. Saag, MD

— It’s the best of times and the worst of times

It seems that we have at least two vaccines with potentially more on the way that might end this pandemic.

There’s finally a light at the end of the tunnel. Don’t get us wrong — the next few months have the potential to be very bad — but the news about the Pfizer and Moderna vaccines demonstrating greater than 90% efficacy is simply huge.

Let’s take a look at what you need to know.

How do these vaccines work?

Both vaccines are messenger RNA (mRNA) vaccines and will be the first mRNA vaccines to ever come to market. Most vaccines are protein-based: we inject a protein from a virus into the body and hope the immune system generates protective antibodies. This is how a flu or hepatitis vaccine works. An mRNA vaccine is a gene-based approach that injects the genetic code of a protein from SARS-CoV-2 into your body so you produce the viral protein and generate an immune response.

There are two major theoretical advantages to having your body produce the antigen directly as opposed to injecting the antigen:

  • mRNA vaccines may stimulate a more effective immune response
  • Manufacturing mRNA vaccines is cheaper and more scalable than protein-based vaccines.

What does 90% efficacy mean?

Vaccine efficacy is a measure of how well a vaccine works when vaccination goes perfectly according to plan. It’s calculated by looking at the risk of infection in vaccinated people compared to risk in unvaccinated people. Efficacy of 90% means that infection rates with the vaccine are 90% lower than seen without vaccination.

When thinking about a vaccine for COVID-19, we care about preventing disease, reducing the severity of disease, and preventing death and disability. In these trials, efficacy of the vaccines was measured by reduction in symptomatic infection. We don’t yet know about the degree of protection from asymptomatic infection (i.e., sterilizing immunity), but we do know that if they reduce symptomatic infection, they should reduce deaths, hospitalizations, and intubations as well.

What are the biggest remaining questions about vaccine efficacy?

While the preliminary data from both trials show that the vaccines reduce symptomatic infections, it remains unclear whether they are preventing asymptomatic infections. This unanswered question will impact the vaccines’ ability to reduce viral spread which is paramount in combating a pandemic.

With COVID-19, we care almost as much about whether the vaccine prevents someone from becoming infectious as we do about whether it prevents someone from becoming infected. This lingering question will be important to follow as additional data becomes available.

Are there any differences between the Pfizer and Moderna vaccines?

The most notable difference has less to do with science and more to do with shipping and logistics. The Pfizer vaccine must be kept at -70°C, which will require dry ice, whereas the Moderna vaccine can be shipped at -20°C and, according to a recent update, stored for up to a month at ordinary refrigerator temperatures. Thus, the Moderna vaccine has a simpler distribution and storage protocol as compared to Pfizer’s vaccine. The reason that both vaccines require cold temperatures is because RNA is an unstable molecule and prone to degrading. Higher temperatures mean more breakdown and thus, we suspect, lower efficacy.

Both vaccines will require two doses with the Moderna vaccine given 28 days apart, and the Pfizer vaccine given 21 days apart.

Other than the differences noted above, the two vaccines appear more similar than different. Both appear to be highly effective based on the initial data, but this may change as more data becomes available. But, the difference in storage temperatures as described above could prove to be an important differentiator moving forward.

What’s left to worry about?

From a vaccine perspective, Pfizer’s and Moderna’s press releases are close to a best-case scenario. We’ve reached a pivotal moment where we feel excitement that an effective vaccine is on the short-term horizon. There is light at the end of the tunnel.

The bad news is that case counts and hospitalizations are rising exponentially and this vaccine will not help us today, tomorrow, or next month. Our biggest fear is that we will let our guard down based on the positive vaccine news. The 2021 vaccine timeline will not meaningfully change the trajectory of the pandemic over the next few months. We should expect things to get worse before they get better.

But the recent vaccine news is an important step toward our shared goal of returning to normal life. Promising vaccine data shouldn’t be seen as a reason to let our guard down but should strengthen our resolve to get through the last few months of the pandemic by wearing masks, physically distancing, and avoiding large indoor gatherings. Winter may be coming, but hope springs eternal and this spring holds even more reason for optimism based on the recent vaccine news.

Gregory Katz, MD, is a cardiologist at the Hudson Valley Heart Center of Nuvance Health working as an intensivist at Vassar Brothers Hospital during the COVID-19 pandemic. He writes an email newsletter on medicine and COVID-19. Harry S. Saag, MD, is a hospitalist and clinical assistant professor of medicine at NYU Langone Health, and CEO of Roster Health. He publishes a blog on COVID-19.

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