the right mix of vaccines, ventilation and masks to be safe-

from Cristina Marrone

With vaccines and the arrival of the highly contagious Delta variant, the scenario in the classroom changes radically. Portable purifiers and microphones for teachers reduce the risk of infection (and so does the ISS)

Returning to school safely is the goal of principals, teachers, students who would like to archive their teaching at a distance and is indicated as a priority on the political agenda. The first bell already rung with a series of rules, from salivary tests to quarantines and still many open problems, but how are things really in the classroom? How can the return to school be guaranteed in the presence of the students? minimizing the risks of coronavirus infection? In fact, it is now known that the transmission of the virus occurs mainly through theaerosol
, the infected droplets that remain in the air maintaining a charge of infectivity for a time long enough to be inhaled by susceptible individuals.

How the two new variables affect: Delta and vaccines

The scenario that awaits us is radically different from last year. There are two new variables: on the one hand we have i vaccines (compulsory for teachers) which have proved to be very effective in preventing serious illness and death but a little less in preventing infection according to various international data, on the other hand, unfortunately, the Delta variant, notoriously more contagious, has become prevalent. There viral load of the Delta variant in the upper respiratory tract from 100 to 1000 times higher than the Wuhan strain and also il incubation time, which at the beginning of the pandemic was 5-8 days now
pi short, about 3-5 days. Furthermore, the viral load of the Delta is more uniform among the infected and there is no variability (high and low viral load) typical of the original strain and of other infectious diseases. All these features made the Delta variant the 60% more transmissible than Alfa which in turn was 50% more transmissible than the strain spread in Europe last year. The American CDCs estimate that a person infected with Delta can transmit the virus to at least 7-9 susceptible individuals.

The crux of the decline in immunity and the impact of the Delta on re-infections

If the Alpha variant were still in circulation, vaccines and more modest mitigation measures would be sufficient to ensure a safe school. The arrival of the Delta variant has for changed (a lot) the cards on the table. Vaccines alone will not be enough because, as can be seen above all in other countries that began the vaccination campaign before Italy, what are called revolutionary infections that is, it infects fully vaccinated people who, in turn, can infect. The reasons are mainly two: over time the immunity induced by the vaccine seems to decrease, which is why the more fragile ones will proceed as early as September with the third dose. Furthermore, as mentioned at the beginning, the vaccine protection from symptomatic infection, especially with Delta, is lower than the protection from serious illness or death. According to the data provided by the Istituto Superiore di Sanit in the latest report in Italy, the vaccines in use protect 96% from death, 93% from serious illness and 77% from diagnosis, although in other countries the protection from infection appears even lower.

No protection is enough by itself

The emission of the Delta variant is so high that in a closed environment, due to airborne transmission, the extremely high risk of contagion if adequate protection measures are not taken. Unfortunately vaccines alone are not enough, as well as ventilation alone or masks. Against the Delta we must deploy all the weapons we have at our disposal, with the right mix of immunized, engineering protection measures and masks recommends Giorgio Buonanno, professor of Environmental Technical Physics at the University of Cassino and author of many works on the airborne transmission of pathogenic respiratory agents.

Adequate ventilation and limits to the emission of the source

How then can the spread of the infection be contained in a closed environment such as a classroom? Vaccines are important, but i children kindergarten and elementary schools and children of the sixth grade they still do not have access to vaccination. It means they will be in class all susceptible. And even in high school it is unlikely that you will get 100% vaccinated. To reduce the contagion, in addition to the masks it would go adequately increased the ventilation and, above all, limited the emission of the source (the infected person), which means making the contagious person less dangerous.

The different scenarios

With the help of Professor Buonanno, who with the team of the University of Cassino, on the basis of a theoretical model published in the scientific literature and included in a recommendation also by the Istituto Superiore di Sanit (here), has developed a tool to estimate the risk of contagion and identifying how to reduce the risk of infection, we have hypothesized different scenarios.

The starting point is one class of 25 students who remain in the classroom for 5 ore with a positive teacher to Sars-CoV-2 that he explains aloud for an hour. The abscissa (horizontal line) shows i air changes per hour where is the value 1 corresponds to bad ventilation, with occasional openings of doors and windows while the value 3 roughly corresponds to keep doors and windows constantly wide open. The ordinate (vertical line) instead shows the percentage of immunized where, in the mathematical model, by immunized we do not simply mean people vaccinated or cured of previous infections, but immune, that is, who will not be contagious (while with the vaccine we have seen that reinfection, albeit infrequent, is possible above all with the Delta).

What happens in the classroom if everyone wears the surgical mask

Let’s assume a 50% of those present immunized (corresponding to more than 60% of the vaccinated class since the number of immunized always lower than the number of vaccinated due to reinfections) and that everyone wears the surgical mask. With a poor ventilation they could infect up to ten students (dark red). Holding the windows always wide open, as suggested by the guidelines (which is equivalent to 2-3 air changes per hour, variable according to the size, opening and position of the windows), the infected would still be 7. Of course, if no one was vaccinated, as in the younger classes, the numbers would be even higher. To be able to obtain a number of infected people less than 1 (Rt<1) the 95% of the students were immune and at the same time the windows remained wide open.

What happens in the classroom if everyone is wearing the FFP2

The situation would improve if instead of the surgical masks students and teachers wore the FFP2, more protective, but objectively difficult to wear for 5 hours. Always considering a 50% of immunized (but you can create scenarios independently by going up or down with the number of immunized) with poor ventilation, up to 5 people could be infected. With the windows always wide open, however, there would still be 3 infections. To obtain a number of infected people less than 1 that should at least 85% of those present were immunized and the windows could always remain open.

Ventilation, mechanical ventilation, purifiers and Co2 meters

And speaking of Open windows, the principals, especially of the schools in the North, are well aware that in the middle of winter you cannot remain motionless at a desk, with your fingers so sore from the cold that even writing is difficult due to objective climatic problems. To obtain an even better aeration result, the ideal would be to rely on a mechanical ventilation system capable of up to about 10 volumes of air per hour (total air change every 6 minutes) but well few educational establishments have it. Alternatively you can opt for i air purifiers, space-saving devices that look like a heater and that need an electrical outlet to work (cost about one thousand euros). The government has allocated funding for the possible purchase of ventilation devices, leaving the principals completely alone to extricate themselves from the thousands of offers from companies that offer their products, which are not always adequate and scientifically tested. The air purifiers must be chosen appropriately and sized according to the risk analysis of the class: this analysis requires specific scientific skills and it cannot be left to improvisation. The best systems for purification are those equipped with filtration, for example HEPA filters, while ionizers, UV-C radiation systems, ozone are not very suitable for school environments, explains Buonanno (here the guidelines of the ISS). To manage natural ventilation, many schools are buying C02 meters that work like a traffic light, signaling the alert value with red when it is necessary to open the windows, which can be closed when the light turns green again. But even in this case – warns the expert – it is necessary to enter the right parameters such as classroom size and number of students and an algorithm that calculates the risk values, nothing left to chance.

Teachers with the microphone to reduce the risk

What else could be done to reduce Sars-CoV-2 emissions? The scientific community has long been telling us that when you go from simple breathing to speaking out loud or singing, the increase in aerosol particles emitted increases by up to 100 times. For this reason, the ISS guidelines suggest teachers, who must keep a high tone of voice to explain, to use microphones, so you can lower your voice and emit less aerosols. And just theuse of the microphone by the teacher, combined with masks and adequate ventilation, represents that right mix, the best case scenario, that could really protect our kids from the Delta.

In detail, with 50% immunized, theuse of the microphone, windows always open (or using portable purifiers to maintain an acceptable climate in the classroom) you would have a Rt tra 1-2. Going up to 70% of immunized with three air changes per hour (with windows or purifiers) would reach an Rt <1 (lighter yellow). Clearly with poor ventilation we would return to a higher risk of contagion, up to 3 individuals

If it were the student who was positive for Sars-CoV-2

So far we have seen the risks of infection for students in the presence of a Sars-CoV-2 positive teacher. What would happen instead if there was a Sars-CoV2 positive student in the classroom? As already stated, more aerosols are emitted when speaking, especially aloud, which is why teachers are considered more dangerous.

Let’s assume that one student is interrogated for 20 minutes. Again, with 50% immunized and poor ventilation about 7 companions would risk infection. With fopen windows they would risk contagion between 4-5 students. Need reach 95% immunized and three air changes per hour to obtain an Rt <1. Equipping the student with a microphone would greatly reduce the risks as seen for teachers. The Delta variant seems to give a lot of trouble but by adopting all the solutions that different sectors of science offer us, even such a complex situation can be kept under control, mitigating the risks of contagion for students and teachers.

September 13, 2021 (change September 13, 2021 | 13:11)


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