COVID-19 infection rates increase when airborne pollen levels are higher

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When airborne pollen levels are higher, increased SARS-CoV-2 infection rates can be observed. These results were determined by a large-scale study conducted by an international team headed by researchers at the Technical University of Munich (TUM) and the Helmholtz Zentrum München.

Members of high-risk groups can protect themselves by watching pollen forecasts and wearing dust filter masks.

In the spring of 2020, the outbreak of the coronavirus pandemic appeared to coincide with the tree pollen season in the northern hemisphere. These observations prompted an international team of researchers to conduct an extensive investigation: The scientists wanted to know whether there is a demonstrable link between airborne pollen concentrations and SARS-CoV-2 infection rates.

Pollen is a significant environmental factor influencing infection rates

Under the leadership of first author Athanasios Damialis, the team at the Chair of Environmental Medicine at TUM collected data on airborne pollen concentrations, weather conditions and SARS-CoV-2 infections – taking into consideration the variation of infection rates from one day to another and the total number of positive tests.

In their calculations, the team also included data on population density and the effects of lockdown measures. The 154 researchers analyzed pollen data from 130 stations in 31 countries on five continents.

The team showed that airborne pollen can account for, on average, 44% of the variation in infection rates, with humidity and air temperature also playing a role in some cases. During intervals without lockdown regulations, infection rates were on average 4% higher with every increase of 100 grains of airborne pollen per cubic meter.

In some German cities, concentrations of up to 500 pollen grains per cubic meter per day were recorded during the study—which led to an overall increase in infection rates of more than 20%.

In regions where lockdown rules were in effect, however, the infection numbers were on average only half as high at comparable pollen concentrations.

Airborne pollen weakens immune response

High pollen concentrations lead to a weaker immune response in airways to viruses that can cause coughs and colds. When a virus enters the body, infected cells usually send out messenger proteins. This is also the case with SARS-CoV-2. These proteins, known as antiviral interferons, signal nearby cells to escalate their antiviral defenses to keep the invaders at bay.

Additionally, an appropriate inflammation response is activated to fight the viruses.

But if airborne pollen concentrations are high, and pollen grains are inhaled with the virus particles, fewer antiviral interferons are generated. The beneficial inflammatory response itself is also affected.

Therefore, on days with a high concentration of pollen, it can lead to an increase in the number of respiratory illnesses. This also holds true for COVID-19. Whether individuals are allergic to the different pollen types is irrelevant.

“You cannot avoid exposure to airborne pollen,” says Stefanie Gilles, first author of the study. “People in high-risk groups should, therefore, be informed that high levels of airborne pollen concentrations lead to an increased susceptibility to viral respiratory tract infections.”

Athanasios Damialis says, “When studying the spread of SARS-CoV-2, environmental factors such as pollen must be taken into account. Increased awareness of these effects are an important step in preventing and mitigating the impact of COVID-19.”

Particle filtering masks provide protection

What can vulnerable people do to protect themselves? Claudia Traidl-Hoffmann, last author and a professor of environmental medicine, advises people at high-risk to monitor pollen forecasts over the coming months. Claudia Traidl-Hoffmann says: “Wearing a particle filtering mask when pollen concentrations are high can keep both the virus and pollen out of the airways.”


Pollen exposure weakens the immunity against certain seasonal respiratory viruses by diminishing the antiviral interferon response. Here we investigate whether the same applies to the pandemic severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which is sensitive to antiviral interferons, if infection waves coincide with high airborne pollen concentrations. Our original hypothesis was that more airborne pollen would lead to increases in infection rates.

To examine this, we performed a cross-sectional and longitudinal data analysis on SARS-CoV-2 infection, airborne pollen, and meteorological factors. Our dataset is the most comprehensive, largest possible worldwide from 130 stations, across 31 countries and five continents.

To explicitly investigate the effects of social contact, we additionally considered population density of each study area, as well as lockdown effects, in all possible combinations: without any lockdown, with mixed lockdown−no lockdown regime, and under complete lockdown. We found that airborne pollen, sometimes in synergy with humidity and temperature, explained, on average, 44% of the infection rate variability.

Infection rates increased after higher pollen concentrations most frequently during the four previous days. Without lockdown, an increase of pollen abundance by 100 pollen/m3 resulted in a 4% average increase of infection rates. Lockdown halved infection rates under similar pollen concentrations. As there can be no preventive measures against airborne pollen exposure, we suggest wide dissemination of pollen−virus coexposure dire effect information to encourage high-risk individuals to wear particle filter masks during high springtime pollen concentrations.

Progress of COVID-19 is presumed to be often asymptomatic or associated with only mild to moderate symptoms, mainly fever and dry cough (1). However, in susceptible individuals, such as elderly persons with metabolic, cardiovascular, and/or pulmonary comorbidities (2), COVID-19 can exacerbate to severe pneumonia requiring oxygen supplementation and intensive care treatment. COVID-19−associated deaths are mainly due to severe acute respiratory syndrome (SARS), cytokine storm (3⇓–5), or disseminated coagulopathy leading to multiorgan failure. According to World Health Organization (WHO) estimates, the overall case fatality rate is 3.4% (6, 7).

SARS coronavirus 2 (SARS-CoV-2), the causative of COVID-19, is a novel member of the Betacoronaviridae family with presumed zoonotic origin (8). It is a positive-stranded RNA virus with a genome size of ∼30 kb (9). SARS-CoV, the agent of the SARS epidemic of 2002 and its closest related sibling, is highly susceptible to antiviral interferons (IFNs) and has developed immune suppression mechanisms on the basis of antagonizing host cell IFNs.

The accessory proteins encoded by the genes ORF3b, ORF6, M, and N of SARS-CoV-2 are highly homologous to their SARS-CoV and Middle East respiratory syndrome counterparts, which are type I IFN antagonists (10). Another set of accessory proteins, encoded by the genes E, ORF3a, and ORF8b and common to both SARS-CoV and SARS-CoV-2, are activators of the NLRP3 inflammasome (11, 12) and have up to 95% interstrain amino acid sequence identity (9). Excessive inflammasome activation and subsequent pyroptosis is the underlying mechanism for the IL-1β dominated cytokine storm associated with SARS-CoV mediated multiorgan failure (4, 13).

A recent, large cohort study from South Korea reported that asthma exacerbations in school-aged children are associated with coexposure to multiple seasonal environmental factors, that is, ozone, rhinovirus, and tree pollen (14). Another study recently reported that pollen grains of various plant taxa release as yet unidentified compounds that down-modulate the production of antiviral λ-IFNs in respiratory epithelial cells, and provided evidence from human and mouse models that pollen exposure leads to enhanced susceptibility to infection with two different respiratory viruses, human rhinovirus and respiratory syncytial virus (15). Also, some pollen types enhance the release of the IL-1 family cytokines IL-1β, IL-18, and IL-33 from epithelial cells in vitro, indicating a role for pollen in NLRP3 inflammasome activation (16, 17). Thus, two mechanisms of the innate immune response, inflammasome activation and antiviral IFN response, appear to be modulated toward the same direction by pollen and SARS-CoV-2.

The first COVID-19 cases were officially reported for European countries at the middle to end of January 2020. On 12 March, the WHO officially announced the onset of a global COVID-19 pandemic, with over 33% of the world´s nations reporting local spreading of the infection.

Around the same time, a large-scale warm spell across the bulk of the Northern Hemisphere initiated the first large seasonal peak in tree pollen emissions. The synchronized timing of the spreading of the infection and the higher pollen concentrations, in combination with the recently found potential of pollen to enhance susceptibility for respiratory viruses, prompted us to analyze whether, under certain weather conditions, a positive correlation between SARS-CoV-2 infections and airborne pollen could be observed.

We therefore collected airborne pollen data from most pollen monitoring stations operating at that time, from a total of 31 countries and from all inhabited continents, including both the Northern and Southern Hemispheres, and investigated for relationships between daily pollen concentrations and SARS-CoV-2 infection rates, also taking meteorological and sociodemographic factors into account.

Our results reveal that the simultaneous exposure to SARS-CoV-2 (via other infected human carriers) and airborne pollen may, under “favorable” weather conditions, promote viral infection. While it is meaningful to inform the public about this risk, the wording should be extremely well considered to avoid misunderstandings and to not cause panic. On the other hand, wide dissemination of the potential dire effects of virus−pollen coexposure ought to be urgently and clearly communicated: As we cannot avoid airborne pollen exposure, high-risk groups have to be informed to wear particle filter masks during the pollen season, especially in springtime.

REFERENCE LINK: https://www.pnas.org/content/118/12/e2019034118


More information: Athanasios Damialis et al. Higher airborne pollen concentrations correlated with increased SARS-CoV-2 infection rates, as evidenced from 31 countries across the globe, Proceedings of the National Academy of Sciences (2021). DOI: 10.1073/pnas.2019034118

Stefanie Gilles et al. Pollen exposure weakens innate defense against respiratory viruses, Allergy (2019). DOI: 10.1111/all.14047

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