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.

2. 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.

3. 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.”

4. 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.

5. 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.