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

Since PCR tests are only available to a small part of the population, Professor Marc Brisson, who leads the research group on mathematical modeling and health economics related to infectious diseases at Laval University, struggles to make predictions about the number of cases that will not lead to hospitalization. However, it is possible that this sixth wave will lead to a large number of asymptomatic, mild to moderate infections due to the increased contagiousness of the virus, as well as the time that has passed since the booster dose. Indeed, according to analyzes in the United Kingdom, its effectiveness against infections that do not result in hospitalization or death falls off rapidly: against BA.2 it drops from 74% two weeks after injection to 46% after eight weeks. In other words, in Quebec, people recalled in early January are almost half as protected from these minor infections today as they were at the peak of the Omicron wave.

In the countries of Europe affected by the sixth wave, estimates of the number of non-serious cases remain two to three times lower than the peaks reached during the fifth wave. This still represents a lot of people who have to be away from work for several days. Will we see the same here? The sharp increase in absenteeism in the health sector does not bode well.

In the last month, the impact of sub-option BA.2, which began to spread in Europe and Asia earlier than in North America, has become better understood. Several recent and rather serious studies, as they relate to a large number of infections caused by BA.2, show that it causes the same symptoms, no more and no less serious, and that it eludes the immune system in much the same way as Omicron. caused by the vaccine. It has been confirmed to be 30-40% more contagious, so it can spread very quickly. These properties were taken as hypotheses in the latest simulation of a possible sixth wave, conducted on March 2 by Marc Brisson and his team and published by INSPQ.

Reasons for concern

The fifth wave had a devastating effect on surgeries and medical checkups, causing many delays, and even a wave two or three times as low would not be taken lightly. Moreover, the ability of the system to take care of patients depends, first of all, on the state of health of its own staff. However, on March 27, D.R Luc Boileau announced that 8,600 healthcare workers were absent due to COVID-19, up 60% in just one week.

The geographic distribution of cases in the general population and among healthcare workers could lead to a greater reduction in workload in some regions than during the fifth wave, which hit Greater Montreal much harder than the rest of Quebec. For example, in Gaspe, there were 25% more hospitalizations in March than in January. “In the United Kingdom, which is currently experiencing a strong sixth wave, we see that London is largely spared, while it was at the epicenter of the fifth wave,” explains Mark Brisson. On the other hand, the number of hospitalizations in Scotland has increased significantly. We could see the same phenomenon happening here with the sixth wave, which is especially painful outside of Greater Montreal. »

If the national director of public health appears to be ruling out the return of certain medical measures for the moment, a possible reduction in the workload in hospitals due to the combined pressure of the number of patients to be treated and absenteeism of caregivers could make a difference.

In their latest simulation, Mark Brisson’s team assumed a marked increase in booster vaccination coverage in March. However, this did not happen: only 60% of already vaccinated adults received it. Although this percentage increases with age and exceeds 90% for those over 70, many older people are still not protected by a booster. This is, for example, the case for 12% of CHSLD residents who received two doses.

We now know that compared to BA.2, as well as Omicron, this dose significantly reduces the risk of hospitalization. A preliminary study of cases in Qatar, where BA.2 took office at the end of December, shows that the effectiveness of two injections against hospitalizations due to any version 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 to three compared to what they are after two doses. In the very elderly, for whom these risks are high, this is no small matter.

Infected with Omicron, protected from BA.2?

Whether they were vaccinated or not, whether they were revaccinated or not, are people infected with Omicron during the fifth wave protected from reinfection with this variant or BA.2? The strongest evidence so far comes from Denmark, as BA.2 appeared there earlier than in many other regions, PCR tests there continued on a large scale, and that the country was hit hard by the fifth wave, during which three Danes were infected. In a preliminary study published in late February, researchers examined 1.8 million PCR-confirmed infections that occurred between November 22 and February 11 in Denmark. In total, only 1,739 people received a new positive test result 20 to 60 days after the initial infection.

A more detailed genomic analysis of these possible reinfections, performed on 263 pairs of samples, showed that 71% of these people were indeed infected twice and that 18% of them had first Omicron and then AD. The victims of these two variants were mostly unvaccinated and young people. None of the reinfections resulted in hospitalization or death.

In other words, in the short term, you really have to be unlucky to get infected with Omicron and then BA.2. However, the possibility of re-infection depends on the circulation of the virus. However, after a strong fifth wave, there was no increase in cases in Denmark, as we have seen for several weeks in other European countries. Therefore, it cannot be ruled out that where the number of cases is currently clearly increasing, the number of re-infections is also higher, especially since this probability also increases over time. “In our model, we assumed that Omicron infection would protect against BA.2, which seems to be mostly true at the moment. On the other hand, perhaps there will be less and less of this protection in order to avoid infections in the coming weeks, which could maintain some circulation of the virus later in the spring, ”said Mark Brisson.

New reminder, is it worth it?

An INSPQ analysis of the hospitalizations that occurred during the fifth wave confirms that the booster dose was very effective in preventing those caused by Omicron, at a rate of 92% for people aged 70 and even 93% for people aged 80 and over. But age, far more than the presence of chronic disease, is a ruthless factor in this variant: after age 60, the rate of hospitalization for COVID (and not just COVID) per 100,000 people increases by 50% every five years, both among the vaccinated and among the unvaccinated. . During the fifth wave, 64% of hospitalizations due to COVID and 95% of deaths were in people over 60 years of age. Clearly, across all age groups, the unvaccinated were more likely to need hospitalization due to COVID.

Due to the current increase in cases, people who have not yet received a booster dose are advised to take it as soon as possible, especially if they were not infected with Omicron or were at the beginning of the fifth wave. What’s more, according to the Quebec Committee on Immunizations, older and more vulnerable people who received a booster more than three months ago should receive a new dose to maintain the robust protection from hospitalizations they need to cope with rising infections. in cases, but also in order to better protect against milder infections in the short term and thus curb infection in their age group. Therefore, in the last few days, elderly people of all ages living in residential buildings are being offered a new booster dose, where the slightest case, even a mild one, causes many harmful disorders. It is also offered to people over 80 who live at home, as well as people with weakened immune systems.

What do the forecasts say?

Since the start of the pandemic, the monthly forecasts made by Marc Brisson and his team have proven remarkably accurate. A possible sixth wave appeared immediately after the forecast was fulfilled in early February for the so-called Greater Montreal region – Montreal, Laval, Monteregi, Lanaudière and Laurentide. The calculations took into account the first announcements about the easing of sanitary standards after the peak of the fifth wave. They predicted that this weakening would not necessarily lead to a new wave. But three unknowns likely had a big impact: the number of people infected during the fifth wave, the possible decrease in the effectiveness of the booster dose in the elderly, and the speed at which BA.2 could replace Omicron.

The researchers updated their forecasts in early March to reflect the easing that came into effect on 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 hospitalizations remained well below what it would have been if immunity had begun to plummet among the elderly who received the third dose, which is one of the signs – among other things – that they are still well protected.

For this new simulation, they also considered what would happen if the BA.2 variant began to dominate Quebec sometime before mid-March. In this case, their forecasts called for a further surge in hospitalizations and deaths in two weeks, with a peak in incidence and hospitalizations in April. In fact, BA.2 would have become dominant around March 24, according to D.R Boileau.

As in their previous predictions, Mark Brisson and his team built two scenarios depending on the frequency of contacts within the population (scenarios that I explained in this text). Conclusion: at best, we risk peaking at 60 new admissions per day during April, and at worst, at 140. This is about half the maximum observed during the fifth wave.

Marc Brisson’s team will not produce a new calculation update due to lack of time to devote to this task, which has forced the research professor to postpone countless other work over the past two years. Right now he’s focused on long-term forecasting models, on what might happen between now and summer and fall. “But we did test to see if the new elements could make our last prediction wrong,” he explains. And so far, everything suggests that we are moving towards an intermediate number of hospitalizations between our two scenarios. »

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