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As you may know, most vaccines can be given safely to pregnant women, while certain vaccines, essentially the ones consisting of “live” viruses, generally are avoided during pregnancy based on a theoretical risk to the fetus. I have put quotation marks around the word “live”, since viruses are not exactly alive; they are on the boundary between living and non-living. Examples of live vaccines that pregnant women are told to avoid include varicella (chickenpox) vaccine, shingles vaccine, and the combined measles-mumps-rubella vaccine (MMR). These vaccines are either weakened versions of the virus that cause the disease, or they are viruses that are similar to the causative viruses but do not cause the disease. Within this latter category is the very vaccine that gave vaccines their name, namely the smallpox vaccine, consisting of the virus called vaccinia, named for the Latin word for cow (vacca).
The vaccine was discovered at the end of the 18th century by Edward Jenner, based on his realization that young women infected with the more mild disease called cowpox would typically become immune to smallpox, without actually getting smallpox. Instead, they would develop a disease called cowpox. This disease gave them pustules on their hands, but, unlike smallpox, not on their faces and without life-threatening disease. Without knowing anything about viruses, this led Jenner to develop a vaccine that contained the live vaccinia virus as the active ingredient. Over the course of less than two centuries, this led to the eradication of smallpox, and so the vaccine has not been given to children since the 1970s. Today, unless you are in the military, or are a researcher who works with the virus, you would not be given the vaccine, but if you are in one of those categories, your pregnancy status will be checked.
Live vaccines carry an advantage in that they can actually spread from those who are vaccinated to others, conferring them with at least some immunity. However, live vaccines generally require more extensive safety testing compared with inactivated vaccines, and researchers are in a hurry to produce a clinically viable COVID-19 vaccine.
At the other end of the spectrum are vaccines made from inactivated, or “killed” viruses, or killed other agents (bacteria or parts of them) that, for one reason or another, cannot reproduce in your body’s cells. Important examples of the latter category that women are encouraged to get during pregnancy is the tetanus/diphtheria/pertussis vaccine (Tdap) if you are due for a booster, and the flu shot, which you must get each year since there are numerous influenza viruses circulating that mutate fairly easily from season to season.
Reading this during the current pandemic, you might be wondering about the vaccine development to confront SARS-CoV-2 (the virus that causes COVID-19) and how it relates to the different types of vaccines that are already in use for other diseases. Live vaccines carry an advantage in that they can actually spread from those who are vaccinated to others, conferring them with at least some immunity. However, live vaccines generally require more extensive safety testing compared with inactivated vaccines, and researchers are in a hurry to produce a clinically viable COVID-19 vaccine. In contrast, inactivated vaccines tend to be weaker in terms of stimulating the immune system to develop immunity. Consequently, they typically require multiple doses, or booster shots, spread out over many months or sometimes longer. This, in turn, lengthens the time needed to test for clinical effectiveness. Thus, if an organization or company has a candidate vaccine against SARS-CoV-2 (as many do), after first testing the vaccine for safety and effectiveness in laboratory animals and then people, the testing for effectiveness in people must be spread out over an extended time period.
In many cases, vaccines cannot be classified as killed because they are produced through genetic engineering rather than by taking an infectious agent and deactivating it with chemicals or other means. But as with vaccines made in other ways, the length of the testing period still depends on how long the immunity takes to develop, how many boosters are needed to trigger the immunity, and how much the boosters must be spread out over time.
To give you an example of one promising COVID-19 vaccine, the company AstraZeneca, has developed what it is calling the Oxford vaccine, since it was developed at Oxford University in the UK. Testing the virus in monkeys, researchers have found that it does not actually keep monkeys from getting infected with COVID-19, but it does protect them from getting extremely sick if they do become infected. Rather than suffering the extreme, life threatening complications in the lungs, the monkeys suffer mild flu and cold symptoms, which is actually like most of the people who get COVID-19 without vaccination. In human trials to evaluate safety, the vaccine was shown to be safe in adults and children, and a much more extensive clinical trial is underway to see how well the vaccine actually works in humans.
Will the vaccine be partially effective as it is in the monkeys? Will it work better than it did in monkeys, actually preventing any symptoms at all? We simply don’t know yet, but the researchers are confident enough of the prospect that the trial will produce good results that they have set things in motion to have 30 million doses available in the UK by September. Later in the year or by early 2021, this number will go up to 100 million doses in the UK, and 300 million doses will be shipped to the United States. So things are moving forward, and you should stay tuned.