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

Since PCR tests are only accessible to a fraction of the population, Professor Marc Brisson, who heads the Research Group in Mathematical Modeling and Health Economics Related to Infectious Diseases, at Laval University, struggles to make projections as to the number of cases that would 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 contagiousness of the virus, but also to the time elapsed since the booster dose. Indeed, according to analyzes carried out in the United Kingdom, its effectiveness against infections that do not lead to hospitalizations or death drops rapidly: against BA.2, it drops from 74% two weeks after the injection. to 46% eight weeks later. In other words, in Quebec, people who had their recall at the beginning of January are almost half as well protected against these non-serious infections today as at the peak 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 maximums reached during the fifth wave. This still represents a lot of people who have to avoid going to work for a few days. Will we see the same here? Soaring absenteeism in healthcare does not bode well.

In the past month, the effect of the BA.2 subvariant has become better understood, as it began to spread earlier in Europe and Asia than in North America. Several recent and fairly solid studies, since they relate to a large number of BA.2 infections, show that it gives the same symptoms, neither more nor less serious, and that it escapes about as much as Omicron to vaccine-induced immunity. It has been confirmed to be around 30% to 40% more contagious, so it has the potential to spread very quickly. These properties are those which had been taken as hypotheses in the last modeling of a possible sixth wave, carried out 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 surgeries and medical examinations, causing many delays, and even a wave half or three times lower would not be taken lightly. Especially since the ability of the system to take care of the sick depends above all on the state of health of its own staff. However, on March 27, the D^r Luc Boileau announced that 8,600 health care workers were absent due to COVID-19, an increase of 60% in one week.

The geographical distribution of cases in the population and among health care workers could lead to greater load shedding in certain regions than during the fifth wave, which had hit greater Montreal much harder than the rest of Quebec. In the Gaspé, for example, 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, whereas it had been at the epicenter of the fifth wave, explains Marc Brisson. Hospitalizations, on the other hand, have increased significantly in Scotland. We could see the same kind of phenomenon happening here, with a sixth wave that hurts especially outside of greater Montreal. »

If the national director of public health seems to exclude, for the moment, the return of certain health measures, a possible load shedding in hospitals under the combined pressure of the number of patients to be treated and the absenteeism of caregivers could change the situation.

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

We now know that compared to BA.2 as to Omicron, this dose significantly reduces the risk of hospitalization. A pre-publication study of cases in Qatar, where BA.2 took over in late December, shows that the effectiveness of two injections against hospitalizations due to either version of the virus continues 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?

Whether they have been vaccinated or not, and whether they have taken the booster or not, are people who contracted Omicron during the fifth wave protected against reinfection with this variant or with BA.2? The strongest data on this subject so far comes from Denmark, since BA.2 appeared there earlier than in many other regions, PCR tests continued there on a large scale. and that the country was badly affected by the fifth wave, during which one in three Danes was infected. In a preprint study published in late February, researchers looked at the 1.8 million PCR-confirmed infections that occurred between November 22 and February 11 in Denmark. In total, only 1,739 people had a new positive test 20 to 60 days after their initial infection.

A more detailed genomic analysis of these possible reinfections, carried out on 263 sample pairs, showed that 71% of these people had indeed been infected twice and that 18% of them had first had Omicron, then BA. 2. Those who fell victim to these two variants were mostly unvaccinated and young. No reinfections led to hospitalization or death.

In other words, in the short term, you really have to be unlucky to be infected with Omicron and then BA.2. The possibility of reinfection, however, depends on the circulation of the virus. However, after a strong fifth wave, Denmark has not experienced a rise in cases as we have seen for a few weeks in other European countries. 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 had assumed that an infection with Omicron would protect against BA.2, which seems to be largely the case for the moment. On the other hand, perhaps this protection will be less and less sufficient to avoid infections in the coming weeks, which could maintain a certain circulation of the virus later in the spring, ”believes Marc Brisson.

A new reminder, is it worth it?

The INSPQ analysis of hospitalizations that occurred during the fifth wave confirms that the booster dose was very effective in avoiding those caused by Omicron, at 92% for people in their 70s and even 93% for people aged 80 and over. But age, much more than the presence of chronic diseases, is a ruthless factor in this variant: over 60, the rate of hospitalization due to COVID (and not simply with COVID) per 100,000 people increases by 50% every five years, both among those vaccinated and among 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 have to be hospitalized because of COVID.

With the current rise in cases, people who have not yet had their booster dose would be well advised to take it as soon as possible, especially if they have not been infected with Omicron, or have been at the start of the fifth wave. Moreover, according to the Comité sur l’immunization du Québec, older and more fragile people who had their booster dose more than three months ago should receive a new one, in order 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 curb contamination in their age group. A new booster dose has therefore been offered for the past few days to seniors of all ages living in residences, where the slightest case, even mild, causes many deleterious disturbances. It is also offered to people over 80 living at home, as well as to immunosuppressed people.

What do the projections say?

Since the start of the pandemic, the projections established each month by Marc Brisson and his team have proven to be remarkably accurate. A possible sixth wave appeared as soon as the projection carried out, at the beginning of 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 relaxation was not necessarily going to translate into a new wave, they predicted. But three unknowns were likely to have a big effect: the number of people infected during the fifth wave, a possible decline in the effectiveness of the booster dose in the elderly, and the speed at which BA.2 could replace Omicron.

The researchers updated their predictions in early March, taking into account the easings effective 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 hospitalizations had remained well below what it would have been if immunity had started to drop sharply among the elderly who received their third dose, one sign – among others – that they were still well protected.

For this new modeling, they also looked at what would happen if the BA.2 variant started to become dominant in Quebec just before mid-March. In this case, their projections called for a further 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 D^r Boileau.

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

Marc Brisson’s team will not produce a new update of the calculations, for lack of time to devote to this task, which has forced the professor-researcher to set aside countless other works for the past two years. He is now focusing on longer-term prediction models, on what could happen between now and summer and fall. « But we still checked whether new elements could make our last projection erroneous, » he explains. And so far, everything suggests that we are heading towards an intermediate number of hospitalizations between our two scenarios. »