Five reasons why COVID-19 cases are increasing when vaccination rates are going up

Canada has reached a significant milestone in its COVID-19 vaccination campaign. As of early August, more than 80 per cent of eligible Canadians had received at least one dose and almost 70 per cent have been fully vaccinated.

Despite high vaccination coverage, COVID-19 cases are increasing in parts of Canada and experts are predicting a fourth wave this fall (or sooner). Other countries that have comparable vaccination rates, including Israel and the United Kingdom, are also seeing a surge of new cases driven by the new delta variant.

Many might be asking: Why are COVID-19 cases increasing when vaccination rates are at their highest levels yet?

  1. We’re unlikely to reach herd immunity

Earlier in the pandemic, scientists touted the idea of herd immunity, claiming that once vaccination rates reached a certain threshold COVID-19 cases would go away. They weren’t wrong but herd immunity is now likely out of reach. That’s because the nature of the pandemic has changed. New variants – including the more transmissible delta variant that is making headlines for being more contagious than chicken pox – has entered the mix.

Herd immunity occurs when the number of susceptible people in the population is low enough that transmission cannot be sustained. It’s related to the reproductive number, called R0, meaning the average number of new infections that a single case of COVID-19 will produce in a susceptible population.

For delta, The New York Times has reported an R0 of five to nine, based on an internal document obtained from the U.S. Centers for Disease Control and Prevention. That number compares to the R0 for the original SARS-CoV-2 virus that is estimated to be between two and three. It’s much higher than other respiratory viruses like seasonal flu or the common cold that typically have R0 of less than two, but still lower than measles, one of the most contagious viruses with an R0 as high as 15.

As of early July, the delta variant comprised more than three-quarters of COVID-19 cases in Canada.

  1. Vaccine coverage is high, but not high enough

Herd immunity also depends on vaccine coverage. As the reproductive number increases, greater vaccine coverage is needed to reach herd immunity.

Canada is among the top countries in the world when it comes to vaccine coverage. But those numbers are calculated only among people who are eligible for vaccination. They exclude the almost 5 million Canadians under age 12 who are too young to be vaccinated. When children are added to the equation, our coverage rates look less optimistic: only 70 per cent with one dose and 60 per cent with two doses.

That’s far below the 90 to 95 per cent threshold likely needed to achieve herd immunity with the delta variant.

  1. Vaccines are not 100% effective

With new variants emerging and vaccination rates increasing, we are seeing a greater number of breakthrough infections, or infections that occur in people who are fully vaccinated. Even though COVID-19 cases are far more likely to occur in unvaccinated people, breakthrough infections are possible, albeit rare.

A recent study from the United Kingdom found that COVID-19 vaccines were only about 30 per cent effective against the delta variant in people who had received a single dose, increasing to close to 80 per cent after two doses. That compares to almost 50 per cent after one dose and 90 per cent after two doses for the alpha variant that was more contagious than the original SARS-CoV-2 virus.

Even with vaccines that are 95 per cent effective, as reported in the original clinical trials, herd immunity is unlikely to be achieved. As the number of vaccinated people in the population increases, so too will the proportion of COVID-19 cases who are vaccinated.

As Lucy D’Agostino McGowan, a biostatistician at Wake Forest University in North Carolina, explains on Twitter: denominators matter.

“If you see something like x% of the sick/hospitalized/deceased were vaccinated, the better the vaccine uptake the scarier this number will seem! It is using the wrong denominator,” writes D’Agostino McGowan. “Instead of looking at the probability of being vaccinated given you are sick, you want to look at the probability of being sick given you are vaccinated.”

  1. Unvaccinated people are fueling transmission, but vaccinated cases can still spread disease

Much of this recent rise in cases can be attributed to low vaccination rates in certain communities – or what some are calling a “pandemic of the unvaccinated” – that is fueling ongoing spread. But these outbreaks in unvaccinated communities can spillover into the broader, largely vaccinated population.

Again, it’s a numbers game. As the number of cases in the community rises, the absolute number of breakthrough infections will increase, even if the overall risk of getting COVID-19 remains low if you’re vaccinated.

When the COVID-19 vaccine trials were conducted, they measured vaccine efficacy against symptomatic disease, not infection or transmission. But new evidence suggests that, when breakthrough infections do occur, fully vaccinated people may equally be as likely to transmit the virus as their unvaccinated counterparts, although they may be infectious for a shorter period of time.

Public health officials recently investigated an outbreak of the delta variant in Massachusetts, finding that fully vaccinated people have similar viral loads – the amount of virus present in the nasal cavity – as unvaccinated cases. Although the study authors acknowledge that their lab tests are only a crude indicator for transmission, their findings prompted the U.S. Centers for Disease Control and Prevention to reinstate its mask mandates for vaccinated Americans.

  1. People are moving around more

Finally, as provinces relax their COVID-19 restrictions, with some provinces opting to remove all restrictions, people are going to interact more. And as mobility increases, so too are cases. That’s because the reproductive number also depends on the number of contacts that you have.

Modelling data from Ontario’s COVID-19 Science Advisory Table shows the impact of easing public health restrictions. Once that province entered its Phase 3 of reopening in mid-July, allowing indoor dining and non-essential retail to resume, the reproductive number inched above one for the first time in more than four months. Whenever R0 increases above one, cases will invariably rise.

As we enter a fourth wave, cases will go up. But this doesn’t necessarily mean a return to life in lockdown. This phase of the pandemic looks different than prior waves, with fewer hospitalizations and deaths and more cases among young people, some of whom are still too young to be vaccinated.

Although we may never reach herd immunity, we will eventually learn to live with this virus. Until then, precautions like masking and social distancing, alongside vaccination, will still be needed to keep cases down.

The comments section is closed.

  • Nola says:

    “even though COVID-19 cases are far more likely to occur in unvaccinated people, breakthrough infections are possible, albeit rare.”
    That is not true. Almost everybody I know who is currently infected is inoculated.

    “Even with vaccines that are 95 per cent effective, as reported in the original clinical trials”
    That “95 per cent effective” is Relative Risk Reduction. Absolute Risk Reduction is ~1% for each of the injections.

    “pandemic of the unvaccinated”
    That is just divisive finger-pointing.

    There is no integrity in this article.

  • Kevin says:

    In what way do these equations factor in prior infections. You say in your fine print captions effective transmission rate is dependent on prior infections but seem to disregard that factor in your equations. Why is this?

  • Jay dub says:

    What a joke article…please please stop shilling and actually report real information both sides.

  • al says:

    in the end there is high chance that unvaxx that survived covid pandemic willl be truely be immune but the failing gov will probably force them too get injected with coercion and force so they can use them as proof that vaccine give a good protection and they were right as if human survived millions of years without theire help but now its VERY VERY important that they need also vaccine and gov to make them heal from a disese since they are smarter than nature and god for sure and if you dont think they are they will destroy your life in the name of theire untested “science” like taxing you cutting your job ensuring you cant pay rent or buy food for your kids this is normal as per them very normal god and normal human immunity does not exist as per them and its “untested” since modern human race is only 300000 year old only THEM can save you and if you dont like it you will be exclude the product they have is 1 to 2 year old bit is very very much for sure better

  • al says:

    doesnt it feel like the info was bullshit when case are 10x higher than pre-vaccine in some area that are now 80-90% vaccinated this make alot of senses are you very proud to force eldery ppl too get injectiin booster that work for 10 week but need 2 week to reach “peak” how can you call this a vaccine? each injection is another portential danger even if this give you boost for a few weeks well as per health engineer the risk vs benefits is very good but they forget the risk vs benefit on long term data like the risk of getting infected during the the small window that the boost give any type of “protection” and surely not any prevention to transmit the disease

  • Greg says:

    The number of Covid19 transmissions and cases will increase as the number of people that are moving around/interacting and not being tested increases. Many countries with high % of vaccination have introduced rules that allow vaccinated persons to move around/travel/attend events without being tested. Unvaccinated persons still have to be tested. We know that vaccination does not prevent the transmission of Covid19, but it does reduce the symptoms, thus making transmission undetectable/invisible. Is it possible that the ‘no need to test if vaccinated’ rule is enabling more rapid undetected transmission? After all, for transmission to increase it must be moving through a larger group. The larger group is the vaccinated group. It is statistically impossible for the minority group of unvaccinated to cause a surge in covid19 infections nationwide/globally that is larger than or equal to the number of infections caused when there were no vaccines/low vaccination-% of the population.

  • Janet Stoddart says:

    Why if cases are rising in both vaccinated and unvaccinated are we not going back to restrictions.
    Do the medics expect this delta variant to decrease? Are they looking at additional research to fight specifically the delta variant with vaccines
    Is this here to stay at such a climb? Does this mean we can never travel safely again.

  • Aple says:

    No mention in here of animal reservoirs. If deer and mink and cats and gorillas and innumerable other animals can catch and spread this, the unvaccinated are not the problem. The unvaccinated will always be a huge proportion of the vulnerable population, no matter how totalitarian we get.

    • Al Neufeld says:

      Why are the cases as high or higher now that roughly 74% have been vaccinated as compared to last December when no one was vaccinated.It just really doesn’t make any sense

  • Eric says:

    It doesn’t explain why cases are higher when 80% of the population should be at least 70%- 80% vaccinated. You’re trying to argue the vaccine works and cases are increase at the same time. How can both these things be true.

    • George says:

      Point #4. These are harsh and unfounded comments that are causing division in our society. If you are going to make points such as these, at least back them up with meaningful data. Show us proof that the unvaccinated are causing this problem. This article is false and misleading.

  • Lisa says:

    Great article to explain some things that are not making sense in my head.

  • Grigori Khaskin says:

    Why I see no analyses for people who had the virus already and recovered. It is very unlikely that they will get the disease a second time. But it is a push to vaccinate these people. What is the reason here? Where is the stat here? I am one of these people, and do not want a vaccine. My immune system is much better

    • Bob says:

      Your immune system is actually far less prepared to deal with another variant. The vaccines offer increased protection. Those that have been infected can easily be infected again and possibly in a worse way. Seriously people, start listening to people who know what they are talking about.

    • Stanley Levinson, Ph.D. Professor of Medicine. says:

      It is likely that you have some immunity if you were infected. More over, if you were naturally infected you should have immunity against other viral proteins besides the spike protein. But infected people show a wide range of spike antibody response with asymptomatic and mild cases showing on the average much lower concentrations of spike antibodies than people with sever disease. On the other hand, people vaccinated with RNA vaccines consistently show spike antibody levels as high or higher than those with severe disease. This variability is reason for concern which has been pointed out by the director of CDC (Dr. Rochelle Walensky) For this reason, I recommended that you get at least one RNA vaccine dose if you are certain you were infected, otherwise, two doses.

      • Nola says:

        All of the data I have seen suggests that inoculation after recovery from infection makes no statistical difference in preventing severe illness, hospitalization, or death. Natural immunity is just as effective (or better) as inoculation.

        You are nobody here’s doctor. You have no right to “recommend” experimental medical treatments to strangers.

  • Rahul More says:

    Really great info about the vaccination. Cleared all myths going in my head. Great article for a health blogger like me. Do visit my health blog . https://www.healthcarenfitness.com/

  • Adelaide Bruechert says:

    When referring to the “unvaccinated”, it is imperative to keep in mind that some of those individuals may have already had the virus, and may still retain immunity. I have noticed that nobody is excluding those individuals from the data, and therefore I believe our numbers are skewed. I recently had my daughter’s antibodies tested 10 months after the virus, and they were still present. I don’t hear of any research on unvaccinated individuals who have had the virus, and their immunity relative to the vaccinated – antibodies as well as T-cells. Where are these comparative studies, and how can we gather quality data if we’re encouraging the ones who have had the virus to get vaccinated, especially if they may still be immune, and perhaps to a better degree than the vaccinated? Poor Science.

  • Bruce says:

    Here’s the number 1 reason, the Liberals policy of allowing international flights, The Sun has published the number of flights carrying infected people into Canada, in July and now August the number is frightening

    • Pilot says:

      Over a million came across the border in the month of May, still coming across, being bussed and flown in. The older ones get right on our Social Security and Medicare, without having paid in or paying for it.

      No forced testing. No forced vaccine.

      So how is there a pandemic??

  • Stone Li says:

    Thank you for your contributions! The point about not 100% effective is so important because people use that as an argument that the vaccine is ineffective…

  • Michael hunter says:

    How is it schools can mandate jockstraps proper shoes and gym uniforms in schools and masks? Why are teachers and nurse allowed to continue to be allowed to work in there professional jobs with out being vaccinated?
    My question is if the governments of the world had done their jobs and mandated all federal employees must be vaccinated would we be ok today?

    • Pilot says:

      Because we are a free nation, not a totalitarian one.
      People go to restaurants, casinos, you name it, right now.
      People are pouring across the border, and no one is testing them, or mandating a vaccine for them.
      So why pick on teachers?

      • Joe says:

        Many running across the border would love to have access to a vaccine. A presumably educated teacher who willfully is an idiot has no business infecting children. In no way are health protocols totalitarian, they are trying to protect people. They are the same thing as OSHA codes and crash testing in cars. Maybe you would be more comfortable without a hardhat but you sure will be grateful for it.

      • David G Martin says:

        No one is “picking ” on teachers. But after all they are dealing with children that cannot be vaccinated. It isn’t a lot to ask for. People who are unwilling to take the simple step of getting themselves vaccinated in order to protect their communities from the coronavirus shouldn’t be around children. Not just because their lack of immunity poses a risk for the community, but also because their mindset of putting themselves before others is not how teaching should be done.


Catharine Chambers


Catharine Chambers is a PhD candidate in epidemiology and Vanier Scholar at the Dalla Lana School of Public Health at the University of Toronto.

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