Vaccination often achieves “herd immunity”, meaning that illiterate people are protected because a disease is no longer spreading in society. However, in the case of COVID-19, despite progress in vaccination, it can never be achieved.

Matthew Perrault
Press

What is mass defense

Initially, a year ago, we were talking about collective immunity in a context where there was no cure or vaccine against COVID-19, recalls epidemiologist Benoit Massé at the University of Montreal. “It was mentioned in Sweden, which did very little imprisonment with the idea of ​​rapidly gaining collective immunity,” Masey said. But eventually, the proportion of people infected in Sweden was also very low. With the great effectiveness of vaccines, we again started talking about collective immunity. As with many infectious diseases, an infected or vaccinated person can no longer catch or transmit the disease. The virus almost stops moving. Therefore, if 80% of the population is immune to the virus, four out of five people will not become ill after coming in contact with the infected person. The group immunity threshold varies greatly from virus to virus: less than 50% for viruses that are less infectious and up to 90% for those that are more infectious. In Quebec, the Legault government estimates that by the end of August a full vaccination of 75% of the population aged 12 and older will receive collective immunity to COVID-19.

Effects of hesitation

The hesitation in vaccination, which prompts some not to be vaccinated, plays a role in the achievement of collective immunity. The vaccination campaign is currently going well in Quebec. For example, in people age 60 and older, the vaccination rate for all doses is more than 85% for the first dose. All over the world, however, some groups have hesitated more about vaccines. “In the United States, everything that happened with Tuskegee’s experience on the treatment of syphilis makes African Americans very suspicious,” says immunization specialist Maryse Guay at Sherbrooke University. Vaccination against COVID-19 is no exception. In the 1930s, the US government recruited 600 African Americans from Alabama to treat a number of illnesses. In fact, he vaccinated him with syphilis, in an attempt to document the natural course of the disease.

Vaccination of caregivers

Another point of concern is vaccination rates below 80% among health workers, or less than 60% in some categories of retirement homes in Quebec. Since April, the Department of Health and Social Services has forced caregivers to undergo tests three times a week to refuse vaccination. “If they are on leave, they have to go through the exam at their own expense,” says D.^Again We have anecdotal data showing that it has influence. That said, vaccination against COVID-19 in the health sector is better against influenza, often less than 50%.

What about travel?

Even if a country has reached the herd immunity threshold, the outbreak of the disease may be due to travel to countries where the virus is still spreading. “Of course, talking about group immunity in the context of a world where we travel a lot is different,” says virologist Andres Finzi of the University of Montreal. For example, a measles outbreak in Quebec in 2011 was linked to a French traveler who has been one of the countries at risk of the disease since 2019, according to the US government’s Centers for Disease Control and Prevention (CDC). Molière for a strong vaccine hesitation in the country. A vaccination passport may limit the risks associated with travel.

Mystery of variants

The immune range of the group increases with the degree of virus infection. However, some variants of the COVID-19 are 50% to 100% more transitive than the original strain, Massey notes. “We were talking about collective immunity with 70% of the vaccinated population, but with variants, it is 80%, 90%. Mr. Finzi also notes that a vaccine-resistant version may appear, which will wipe out the herd’s immunity. “At the moment, variants are resistant to only one layer of the immune response, so vaccines are just as effective. But we cannot say whether it will continue like this. However, when the virus spreads less due to stronger vaccination, there are fewer new types. “That’s why globalization of vaccination is really important,” says Finzi.

Measuring the effectiveness of a vaccine

A ray of hope, American researchers are working on a test that will predict the immune response to a vaccine. “That way, we can know immediately whether it works,” Massé said. And we can give another vaccine to a patient who is not safe. American researchers are comparing the levels of different antibodies in people receiving Moderna vaccines to see if they differ from those who later had COVID-19 and who did not become ill.