Dear Editor,
Recently, many people have been asking me, “What is the point of getting vaccinated if you still can get infected with the Coronavirus?” The short answer to this question is that breakthrough infections (infections in vaccinated people) are rare and, you need to get vaccinated because vaccine induced immunity (which allows you to fight off the virus if infected) is more robust than immunity gained from natural infection (getting sick and recovering), vaccines allow you to develop immunity without risking hospitalization and death and, the trend seen in places with high vaccine coverage with millions of doses administered is that hospitalization and death from COVID-19 favours the unvaccinated. The long answer to this question requires a discussion centred around the Delta variant hence this letter. For basic science surrounding this topic, more on the Delta variant and the important difference between infection and immunity see my letters on Jun 1, 5, 27.
The Delta variant has changed the course of the pandemic in significant ways. It seems to be the cause of a greater number of breakthrough infections than the other variants of concern. However, the present situation remains that vaccinated people are a lot less likely to get COVID-19 than the unvaccinated. But why are there more breakthrough infections now than earlier in the pandemic? Earlier in the pandemic we didn’t have the highly contagious Delta variant as the dominant form. It is the fittest form of the virus, to date, with some ability to evade antibodies (defense molecules that neutralize the virus) and it is 2-3 times more contagious than the original form; now estimated to be as contagious as chickenpox. These properties make it a formidable enemy and is responsible for surges in COVID-19 cases in places with low vaccination coverage.
Researchers found that the Delta variant has a very high viral load – people infected with it had 1000 times more virus particles in their respiratory tract than the original strain. If you view your immune system as a military that has come under attack with a greater number of more fit soldiers and better artillery than expected, then defenses may be penetrated. So even if our immune system is trained for a fight (with vaccines), super high viral loads can make that challenging; even more challenging if you’re unvaccinated. These high viral loads are why control measures are still very much needed like wearing a mask and, when indoors in public spaces it is best to make sure you are in a well-ventilated environment. The high viral loads in some vaccinated people also make them infectious (they can spread the virus) hence the re-instatement of the mask mandate in public spaces by the CDC in the US. So noteworthy, is that the vaccinated, although protected and will fight off the virus in most cases if infected, can be part of the transmission chain so everyone (vaccinated and unvaccinated) still needs to wear a mask and adhere to other control measures to help break these transmission chains – until the vaccine coverage is a lot higher (ideally high enough for population immunity). Further, although the high viral load in the respiratory tract of the vaccinated can make them infectious, this lasts for a shorter period of time than in the unvaccinated since they (the vaccinated) have vaccine-induced immunity and protection allowing them to subsequently fight off the virus.
So, most importantly, these breakthrough infections do not mean the vaccines are not working. Actually, vaccines are working as they should. We know they are not 100% protective but neither is the smallpox vaccine but we have eliminated smallpox with global vaccination campaigns. They don’t prevent infection but allow us to develop immunity, so if infected, we are protected against a severe form of the disease. Most breakthrough infections in people produce a mild form of the disease protecting against hospitalization and death. This is what we are seeing in places like Canada, the UK and US where the vaccine coverage is high enough to have a large amount of data to evaluate. For example, data made publicly available by Health Canada shows that for 615, 893 confirmed COVID-19 cases most of the fully vaccinated people were protected from severe outcomes compared to the unvaccinated (see table taken directly from Canada COVID-19 Weekly Epidemiological Report 18 July to 24 July 2021).
So even with breakthrough infections vaccination is the best protection against the virus. Immunity with vaccination is still excellent at fighting off the virus before the virus invades the lungs and brings on a severe form of the disease. Researchers (including those involved in the development of the Moderna vaccine) recently found that in monkeys lower levels of COVID antibodies were needed to prevent infection in the lungs than the upper respiratory tract (K.S. Corbett et al. 10.1126/science.abj0299 (2021)). This supports, biologically, that the vaccines are very good at protecting against severe disease (in the lungs) than blocking infection entirely; what we have been seeing in real world vaccination data. Vaccines are working as they should and doing a really good job at preventing COVID-19 from becoming lethal and beyond our control in places with high vaccine coverage. They work best at community not individual levels, so, we need most vaccinated. This would bring us closer to population level immunity where we would be able to fully suppress the virus to the point where person to person transfer is unlikely. It would also reduce the number of hosts susceptible to infection. Remember the virus needs us to multiply during which a virulent form can emerge. To prevent the emergence of something more virulent we need most vaccinated faster than the virus can infect, spread and become something worse which means we need to get those vaccines in our arms, get both shots for full protection (where appropriate) and break chains of transmission (in both unvaccinated and vaccinated) by rigidly adhering to control measures. Hopefully the Delta variant is the fittest form of the virus but that’s up to us. Get vaccinated!
Sincerely,
Jacquelyn Jhingree, PhD.
