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In an ideal world, a pandemic vaccine could be delivered in a single shot, so supplies could be stretched to cover a lot of people. It would trigger no side effect more significant than a sore arm. And it would be easy to ship and store.

Unfortunately, this is not an ideal world — not yet, anyway.

For now, the good news is that the United States has two Covid-19 vaccines that have been shown to be highly effective.


What follows is a head-to-head comparison of the ones developed by Pfizer and its partner, BioNTech, and by Moderna. Note that the chances of most individuals being able to “pick” one or the other are slim to none, especially in the initial rollout. The vaccine available is the one you’ll get.

Target population

The Pfizer emergency use authorization is for people aged 16 and older. Moderna’s is for people 18 and older, though the company has recently begun testing its vaccine in 12- to 17-year-olds.


Vaccine efficacy

Both vaccines have shown astonishing — and essentially equivalent — degrees of efficacy, at least in the early stages after vaccination. How they stack up over the long term remains to be seen.

The Pfizer vaccine showed efficacy of 95% at preventing symptomatic Covid infection, measured starting from seven days after the second dose was administered. The vaccine appeared to be more or less equally protective across age groups and racial and ethnic groups.

The Moderna vaccine was 94.1% effective at preventing symptomatic Covid-19, measured starting from 14 days after the second dose. The vaccine’s efficacy appeared to be slightly lower in people 65 and older, but during a presentation to the Food and Drug Administration’s advisory committee the company explained that the numbers could have been influenced by the fact there were few cases in that age group in the trial. The vaccine appeared to be equally effective across different ethnic and racial groups.

Both vaccines seemed to reduce the risk of severe Covid disease. It’s not yet known if either prevents asymptomatic infection with the SARS-CoV-2 virus. Nor is it known if vaccinated people can transmit the virus if they do become infected but don’t show symptoms.

Number of doses/amounts of vaccine per dose

Both the Moderna and the Pfizer/BioNTech vaccines require two shots: a priming dose, followed by a booster shot. The interval between Moderna doses is 28 days; for the Pfizer vaccine, it’s 21 days.

Each dose of Pfizer’s contains 30 micrograms of vaccine. Moderna went with a much larger dose of vaccine, 100 micrograms. It means it is using a little more than three times as much vaccine per person as Pfizer is. And yet, they aren’t getting better results.

Side-effect profile

In the vernacular of vaccinology, vaccines that trigger a range of transient side effects in a lot of recipients are known as reactogenic.

Both these vaccines — in fact, most if not all the Covid-19 vaccines that have reported data so far — fall into the reactogenic category. The Advisory Committee on Immunization Practices, an expert panel that helps the Centers for Disease Control and Prevention set vaccination policies, has advised hospitals they may want to stagger vaccinations among staff — for instance, don’t vaccinate all the emergency room staff at the same time — in case some feel too unwell to work the day after being vaccinated.

The most common side effects are injection site pain, fatigue, headache, muscle pain, and joint pain. Some people in the clinical trials have reported fever. Side effects are more common after the second dose; younger adults, who have more robust immune systems, reported more side effects than older adults.

To be clear: These side effects are a sign of an immune system kicking into gear. They do not signal that the vaccine is unsafe. To date there are no serious, long-term side effects associated with receipt of these vaccines, which will be closely monitored as their use expands.

There have been a handful of reports of people having allergic reactions to the Pfizer vaccine since its rollout began. Those reactions — anaphylaxis or a less severe allergic reaction — were not seen in the clinical trials. It remains to be seen if similar events will be seen with the Moderna vaccine.

Safety for those who are pregnant or lactating

Neither of the vaccines has been tested in these two groups.

Moderna has completed animal studies the FDA demanded of manufacturers; these studies look for evidence that the vaccine might harm the pregnancy or the developing fetus. The company said it saw no such signals.

Pfizer has only interim data from its animal studies, but said it saw no concerning signs either.

The authorizations for both vaccines state there are not enough data to gauge whether they are safe for pregnant or lactating people. FDA’s Peter Marks, director of the Center for Biologics Evaluation and Research, said Friday that those who are pregnant should choose whether to be vaccinated after discussing the risks and benefits with their physicians.

Storage requirements

Both of these vaccines require an elaborate cold chain, the term used to describe the conditions under which vaccines must be stored during distribution and when they are in the doctors’ offices, pharmacies, or public health clinics where they’ll be administered.

But the Moderna vaccine will be far easier to use than Pfizer’s. For starters, Moderna’s must be shipped at -4 Fahrenheit; Pfizer’s must be shipped and stored at -94 Fahrenheit. The former is the temperature of a regular refrigerator freezer; the latter requires special ultracold freezers. Doctors’ offices do not have ultracold freezers; neighborhood pharmacies don’t either.

After thawing, a vial of the Pfizer vaccine must be used within five days; Moderna’s is stable at fridge temperature for 30 days and at room temperature for 12 hours.

Minimum purchase order

The ultracold storage requirement is not the only challenging aspect of the Pfizer vaccine. The minimum amount of vaccine a location can order is 975 doses. A large teaching hospital might need several of those. But there are plenty of places across the country that don’t need 975 doses to vaccinate the people currently eligible for vaccination — health workers and nursing home residents. This is the vaccine that needs to be kept at -94 F. The minimum order size will limit the locations in which this vaccine can be used.

The Moderna vaccine’s minimum order is 100 doses, a much more manageable number.

The Pfizer vaccine is shipped in five-dose vials; Moderna’s vaccine is shipped in 10-dose vials.

Durability of protection

Figuring out how long the protection provided by either of these vaccines will last will take time. It’s going to involve periodic blood draws from some volunteers to see what their antibody levels look like, though a decline in antibody levels doesn’t necessarily equate to loss of protection.

But a large part of this work will involve watching for reports that people who were immunized are starting to contract Covid in larger numbers, a development that would probably lead to recommendations to give people booster shots at some yet-to-be-determined interval.

Correction: An earlier version of this story incorrectly stated that the ultracold freezers being used for Pfizer’s vaccine need to be topped up with dry ice every five days.

  • I just dont understand how a new virus comes upon the world and everyone is scrambling to find a vaccine and usually this takes years if at all even possible to find a vaccine but not 1 but 2, even 3 or 4 companies ALL figure out a vaccine, all different vaccines but ALL work and they figure out this vaccine ALL AT THE SAME TIME!!! They all just happen to find it at the exact same time but theres no cure for the common cold, aids, herpes, cancer, Parkinson’s, alzheimers and many may more… my question is: how could this possible be?

    • Hi Judy,

      If we found the vaccine so quickly, it’s mostly a question of the enormous resources allocated to the laboratories, both in money and people.

      States have signed astronomical contracts with laboratories to speed up the research process and given the damage done by Covid many people have volunteered to participate in efficacy and side effect studies.

      Also know that the vaccines were developed in only few hours from January when we discovered the genome, also laboratories like Moderna and BioNtech have been working on messenger RNA technology for more than a decade and had proven this technology against diseases such as Ebola.

      So the vaccine was already created in January, the clinical trials started in January and they had a very large number of participants, as you know, there are three phases of trials, which Under normal circumstances includes at most 10,000 people tested, but for the Pfizer and BioNTech vaccines there were more than 30,000 people tested.

      Regarding the duration of these trials, an accelerated process has been put in place to ensure that the next phase can begin as soon as we have the results of the previous phase and we know that it is safe. If there were any doubts about safety, the trials were immediately stopped to investigate the dangers, which is what happened with the AstraZenetica vaccine.

      Regarding vaccine release dates, you should know that there are more than 100 vaccines being tested, and at the moment only three of them are safe, and several more are likely to be released over the next month and year. So I don’t think it’s fair to say that all the vaccines have arrived at the same time.

      Finally, regarding the other diseases you mentioned, you should know that these are different issues.
      Common Cold and herpes are viruses that mutate a lot and for which it is very difficult and expensive to make a vaccine, knowing that these diseases are not as dangerous for health and destructive for the economy as the covid is, we are not going to spend a lot of money on them, in fact we always have to study the risk/benefit balance before embarking on such projects, it is useless to spend taxpayers’ money so heavily on diseases that do not cause a major problem.
      Concerning Cancer, we know that research is also moving very fast on this, but it remains a complex disease many parameters has to be taken into account, it is not a simple virus to eliminate, it’s your own body that will produce cancer cells, but you should know that there are already vaccines for some cancers and a new technology has just been discovered, CRSP-9, which will revolutionize the treatment of cancer.

      As for Alzheimer’s and Parkinson’s, these are degenerative diseases, the body deteriorates itself, it is much more difficult to manage this kind of disease because it is related to the damage of time, and probably that they are hereditary. So it’s a whole other issue that has nothing to do with immunology.

      In the case of AIDS, it’s a question of resources.
      Unfortunately, AIDS affects mostly non-Western countries, because in the West we now manage to control this disease. We know how to avoid transmission and we have the tri-therapy to contain it. The most affected countries have neither the means nor the infrastructure to fight against this disease. If the search for this disease continues it has much less money because it’s doesn’t impact us the way covid-19 does, nevertheless the research is promising and there is multiple vaccines in progress.

  • Very informative article. However, can you explain why such a disparity in the storage. Is it because of the makeup of the vaccine? Am I reading to much into this?

    • My understanding is that it has to do with the composition of the lipid nanoparticles. The recipes for the two are proprietary information, but the ingredients are listed. Moderna (mode RNA) works on nothing but RNA drug and vaccine development and has more experience in formulating the lipid nanoparticles as well. There is a wide range lipids to chose from and these lipids are stabilized with a combination of emulsifiers. My guess is that Moderna probably has a better formula — one that can stay stable under higher temperatures. Also, I suspect that the size of the dose (almost 3X Pfizer’s) is a clue to a larger particle protecting the mRNA strand. Hope this helps.

  • The comment about needing 3 times more with no additional benefits is an erroneous statement. Different formulations would require different amounts. moderna’s formulation does not require the refrigeration that the Phizer does, that should tell you something. Your comment leads to incorrect assumptions. You need to improve your reporting and be more responsible.

  • I have heart disease as well every winter I get double pneumonia and have to be hospitalized.
    I’m 56, can I get a Emergency shot of the vaccine?

    • Call/e-mail your Doctor, that is what I have done. Got answer today, and they are still waiting on the shipment and of course first responders will get it first. As to which one Moderna or Phizer that is a crap shoot. Make contact with your Doctor, make shure he is to get you on the list for your condition…..

  • Congratulations on your superb article comparing the two currently available COVID-19 vaccines. As someone who has been involved in laboratory research for many years, I was surprised to read that ultra-cold freezers need dry ice. In my experience, such freezers (-80 C) are driven by electricity and don’t need dry ice. The dry ice is only used for transport to and from ultra-cold freezers. This is a relatively minor detail that few people might notice. Otherwise, kudos on your very informative article! RJ

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