COVID-19: how big will the sixth wave be?

Since PCR tests are only accessible to a fraction of the population, Professor Marc Brisson, head of the research group in Mathematical Modeling and Health Economics Related to Infectious Diseases, at Laval University, has struggled to make predictions about the number of cases that not lead to hospitalizations. However, it is possible that this sixth wave will lead to a large number of asymptomatic, mild or moderate infections due to the increased infectivity of the virus, but also due to the time elapsed since the booster dose. Indeed, according to analyzes conducted in the United Kingdom, its effectiveness against infections that do not lead to hospitalization or death decreases rapidly: against BA.2, it drops from 74% two weeks after injection to 46% eight weeks later. In other words, in Quebec, people who had their recall in early January are nearly half as protected from these non-serious infections today as they were at the height of the Omicron wave.

In countries across Europe affected by a sixth wave, estimates of the number of non-serious cases for it remain two to three times lower than the maxima reached during the fifth wave. This still represents a lot of people who have to avoid working for a few days. Do we see the same here? The rising absenteeism in healthcare does not bode well.

In the past month, the effect of the BA.2 subvariant has been better understood, as it began to spread in Europe and Asia earlier than in North America. Several recent and quite solid studies, as they cover a large number of BA.2 infections, show that it produces the same symptoms, no more or less severe, and that it escapes vaccine-induced immunity about as much as Omicron. It has been confirmed to be about 30% to 40% more contagious, so it has the potential to spread very quickly. These properties are those taken as hypotheses in the latest modeling of a possible sixth wave, conducted on March 2 by Marc Brisson and his team, and published by the INSPQ.

Reasons to worry

The fifth wave had a devastating effect on operations and medical examinations, causing many delays, and even a wave half or three times lower would not be taken lightly. Especially since the system’s ability to care for the sick depends primarily on the health of its own staff. However, on March 27, the Dr Luc Boileau announced that 8,600 health workers were absent due to COVID-19, a 60% increase in one week.

The geographic distribution of cases among the population and among health professionals could lead to greater burden-shedding in certain regions than during the fifth wave, which hit greater Montreal much harder than the rest of Quebec. In the Gaspé, for example, there were 25% more hospital admissions in March than in January. “In the UK, which is currently going through a strong sixth wave, we see London largely spared, while being at the epicenter of the fifth wave,” explains Marc Brisson. In contrast, the number of hospital admissions has increased significantly in Scotland. We could see the same kind of phenomenon happening here, with a sixth wave hurting especially outside of Montreal. †

If the national health director seems to rule out the return of certain health measures for the time being, a possible tax cut in hospitals under the combined pressure of the number of patients to be treated and the absenteeism of health care providers could change the situation.

For its most recent modeling, Marc Brisson’s team assumed a marked increase in vaccination coverage with a booster dose in March. However, this did not happen: only 60% of already vaccinated adults got it. Although this percentage increases with age and is over 90% for those over 70, many seniors are still not protected by a booster. This is the case, for example, in 12% of residents of CHSLDs who had received two doses.

We now know that compared to BA.2 for Omicron, this dose significantly reduces the risk of hospitalization. A pre-published study of cases in Qatar, where BA.2 took over at the end of December, shows that the effectiveness of two injections against hospitalizations from both versions of the virus remains from 70% to 80% over time, and is more than 90% after the booster. In other words, after three injections, the risks of hospitalization are divided by two or three compared to what they are after two doses. In very old people, for whom these risks are high, this is not a detail.

Infected with Omicron, protected against BA.2?

Are people who contracted Omicron during the fifth wave protected against reinfection with this variant or with BA.2, whether they have been vaccinated or not, and whether they have used the booster or not? The strongest data on the subject to date comes from Denmark, as BA.2 appeared there before many other regions, PCR testing continued there on a large scale, and the country was badly hit by the fifth wave, with a three Danes was infected. In a preprint study published in late February, researchers looked at the 1.8 million PCR-confirmed infections that occurred in Denmark between November 22 and February 11. In all, only 1,739 people had a new positive test 20 to 60 days after their first infection.

A more detailed genomic analysis of these potential reinfections, performed on 263 sample pairs, showed that 71% of these people were indeed twice infected and that 18% of them had first Omicron and then BA. Those who fell victim to these two variants were mostly unvaccinated and young. No reinfections resulted in hospitalization or death.

In other words, in the short term you must be really unlucky to get infected with Omicron and then BA.2. However, the possibility of reinfection depends on the circulation of the virus. However, after a strong fifth wave, Denmark has not experienced an increase in the number of cases, as we have seen in other European countries for a few weeks. It cannot therefore be excluded that where the number of cases is currently clearly increasing, the number of reinfections is higher, especially since the passage of time also increases this possibility. “In our model, we assumed that infection with Omicron would protect against BA.2, which seems to be largely the case at this point. On the other hand, this protection may be less and less sufficient to prevent infections in the coming weeks, which can maintain a certain circulation of the virus later in the spring,” says Marc Brisson.

A new memory, is it worth it?

The INSPQ analysis of hospital admissions that occurred during the fifth wave confirms that the booster dose was highly effective in avoiding those caused by Omicron, at 92% for those over 70 and even 93% for those over 80. But age, much more than the presence of chronic diseases, is a relentless factor in this variant: Over the age of 60, the number of hospitalizations from COVID (and not just COVID) per 100,000 people increases by 50% every five years, both among those who are vaccinated and those who are not. During the fifth wave, 64% of hospitalizations due to COVID and 95% of deaths occurred in people over the age of 60. In all age groups, the unvaccinated were obviously much more likely to be hospitalized because of COVID.

With the current increase in the number of cases, people who have not yet had their booster dose would do well to take it as soon as possible, especially if they are not infected with Omicron or have been at the beginning of the fifth wave. In addition, according to the Comité sur l’immunization du Québec, older and more vulnerable people who received their booster dose more than three months ago should be given a new one to maintain the solid protection against hospitalizations they need to cope with the increase. in cases, but also to be better protected in the short term against milder infections and thus to curb contagion in their age group. Therefore, in recent days, a new booster dose has been offered to seniors of all ages living in institutions, where the slightest, even mild case, causes many harmful disorders. It is also offered to people over 80 who live at home and to people with a weakened immune system.

What do the forecasts say?

Since the start of the pandemic, the forecasts that Marc Brisson and his team prepared each month have proved remarkably accurate. A possible sixth wave appeared as soon as the projection was carried out in early February for the so-called Greater Montreal region – Montreal, Laval, Montérégie, Lanaudière and Laurentides. The calculations took into account the first announcements of the relaxation of health instructions, after the peak of the fifth wave. This easing would not necessarily translate into a new wave, they predicted. But three unknown factors would likely have a major effect: the number of people infected during the fifth wave, a possible decrease in booster dose effectiveness in the elderly, and the rate at which BA.2 could replace Omicron.

The researchers updated their forecasts in early March, taking into account the easing that will take effect from March 14. They found that the number of people infected during the fifth wave was high, including one in three in the greater Montreal area. On the other hand, the number of hospital admissions was well below what it would have been if immunity had fallen sharply among the elderly who received their third dose, in part a sign that they were still well protected.

For this new modeling, they also looked at what would happen if the BA.2 variant became dominant in Quebec just before mid-March. In this case, their forecasts called for another spike in hospitalizations and deaths two weeks later, with cases and hospitalizations peaking in April. In fact, the BA.2 would have become dominant around March 24, according to the Dr Boileau.

As in their previous projections, Marc Brisson and his team built two scenarios based on the number of contacts within the population (scenarios I have explained in this text). Conclusion: At best, we risk a peak of 60 new hospitalizations per day in April and at worst 140. That is about half the maximum observed during the fifth wave.

Marc Brisson’s team will not update the calculations again, due to lack of time to devote to this task, which has caused the researcher professor to set aside countless other works over the past two years. He now focuses on longer-term forecasting models, on what might happen between now and the summer and fall. “But we still checked whether new elements could make our last projection incorrect,” he explains. And so far, all indications are that we are heading for an intermediate number of hospitalizations between our two scenarios. †

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